I'll throw a nickle into the conversation...
I was a former Combat Engineer WO, and was commissioned to Lt at the age of 33. I field many questions on the topic similar to yours, so i'll contribute in an effort to help you and anyone else considering this. This is the view from my rose coloured glasses, so please, no one fire a missile at me if you don't agree. In other words, your mileage may vary.
There are 3 distinct ways to CFR (as a Combat Engineer anyway) IMHO:
As a Sgt, you will most likely be required to complete some portions of your Phase trg; 3 and 4 for sure. This has some "ups" and "downs". The downs are obvious. You will find yourself doing alot of training you may have already covered at some point, but as an older person. If you have a family, it will have the usual impacts. Having discussed this with a few I know that went this route, the "ups" are many. You will integrate and gain credibility with your new peers, and hopefully have some time between courses to get some PD finished. If you are really lucky, you may get to pick up some Second Language trg.
As a WO, you will be PLAR'd your Phases and most likely be employed in a Tier 1 position, ie Troop Commander. In my case, this was my entry plan. Loosely put, I was promoted to Lt while at an ERE posting, and shortly thereafter, was posted to Regt as a "new" Lt. Literally, WO one day, Lt the next. Zero crses or trg.
As an MWO, it can be even more tricky. You will most likely move into a Tier 2/2.5 Capt position (Sqn 2 I/C, Ops O, Adjt etc). The water can become much more murky if your intent is to work with the Troops as a Line Officer. Its much less clear if you will become a Staff Officer or a Line Officer when you start at the Tier 2 level. I have seen many go either direction, some by choice, some not.
There are quite a few things that I didn't thoroughly consider when I made the leap:
Age. Although its not policy, nor written anywhere, age becomes your enemy right off of the bat. As a 33 year old WO with 15 years in, the world is your oyster so to speak. As a 33 year old Lt, you notice that there exists unwritten mile-stones about where you should be in a particular age group. This was explained to me in certain detail after I switched over. Generally speaking, a 40 something year old OC is not what is envisioned. Age isn't the be all and end all, but you begin to feel it having an impact, whereas as an young NCO you find yourself wondering what to do with all the time you have left to CRA.
Integration. Another age related issue is in regards to integration with your new peers. In my case, i'm easily 10 years older than my new peers. Again, not a show stopper, but those mornings after the beer funnel with the rest of "subbies" tends to hurt a bit more. I know many NCOs who flat out refused to participate in Mess activities. As an officer, it is not really an option starting out. Another issue is, if you bypass Phase Trg, you need to make a reasonable effort to integrate with your new peers at the Unit. Its hard to describe that feeling you get when you arrive as a newly minted Lt, and when you look around the Unit and the only people you really don't know are your new peers. This is an area where CFR's can get stuck in the past. I'm not saying that you will need to write off old friends and go party with the Lt's, but if you can't let go to some extent, it will become difficult.
Professional Development. OPME and AJOSQ are your little demons to deal with. Although they are not extremely difficult, they will consume a great deal of your spare time. I hear many NCO's complain about ILQ DL essays etc. ILQ DL suddenly becomes comical compared to the reading and writing you are required to do for the History, Leadership & Ethics OPME, and a good portion of AJOSQ. Although much of the "Big Hand, Small Map" stuff in AJOSQ feels like a review having lived it at the tactical end as an NCO, you really do need to understand many of the details behind the voodoo/magic in order to pass AJOSQ. Lastly, all of this needs to be completed on your time, and when you start it, there does not appear to be any light at the end of the tunnel.
Performance/Potential. One of the biggest changes i've noticed is the way officers are evaluated. As an NCO, performance seemed more important in determining the potential of a given individual. If an NCO was missing some key trg to develop his potential, it was generally provided, unless he/she didn't possess the personal attributes. As an officer, yes performance is crucial, but potential, ie Second Language Profile and Post Secondary Education become key in determining potential as you hover at the Capt to Major hurdle.
Second Language Profile. If you do not have one, you find yourself (like I do) wishing you did. As an NCO, if you were selected for employment requiring it, generally it was granted, ie up and coming RSM etc. As an officer, a great deal of onus is put on you to develop a plan to get a Second Language profile.
Post Secondary Education. Although not a huge impediment at the get go, if you see yourself as a LCol some day, start working on it. Without it, you will certainly cease progression as a Maj. It becomes a big mental exercise trying to determine if getting a Degree "is in the cards". Having it will also not guarantee you making it LCol (refer to the age comment).
Being a Pl/Tp Comd. I can't vouch for the Infantry, but in the Engineer Corps as an NCO, there are not many other NCO's you won't have already worked or trained with at some point. This is a double edged sword for both you, and your new Tp/Pl leadership. When everything lines up, its like magic. When some things don't, it can be painful. Having said that. What a great time. You will find yourself doing many of the things you desperately wanted to do as an NCO. I can't say enough about it..... But the reality will soon set in that your days are numbered.
I'll close with this thought. I CFR'd because I was a very young WO and spent much time wondering what the future would hold for me as an NCO. I had 22 years left on the clock. It makes you think. The more I looked around, the more being a WO and beyond seemed like social work, and that I was slowly being disconnected from the Troops. I saw becoming an officer as a whole new opportunity to reconnect and begin a new career of sorts. As a Lt Tp/Pl Comd, you will definitely get that chance, and what a reward it is. Sadly, it is short lived. Once you have completed your time, you will quickly be moved over to another position so the next guy can have his go at it. Your only other chance is as an OC. If you consider my points above, getting that chance to do it again as an OC is quite a bottle neck. Not impossible, but it will require a great deal of effort and may feel like your chasing the unreachable carrot. In other words, you may find yourself as a Capt asking the same questions you did as a WO, "What am I going to do until CRA?". No doubt my pension will most likely be a bit better, and the posting options are ten-fold that of an NCO. Life after Line Command doesn't seem too bad really.
So, would I do it over again in exactly the same manner? That is difficult to answer, and at this point I think its too early to tell. But, If I could turn back the clock, I would go the UTPNCM route without question.
So, having said all of that, before you take the plunge, ask yourself where you "realistically" see yourself being, then consider some of the points above.
