You typically won't need to be in DEU to report in. However, it is best to err on the side of caution and don't assume that reporting-in dress is work dress. You can't go wrong with Salt & Peppers...meaning black wool pants, parade boots, short sleeve white shirt and peak cap. You can wear your sweater with this if it is cool. Or you can subsitute your long sleeve white shirt and tie for the s/s shirt.
If you are still in doubt, check out your paperwork and see if there is a contact name and number. Give them a call and ask what you should be wearing.