Gents,
Destroyer vs Frigate is more a function of armament and capability than it is of weight and dimension. The Destroyer in our case has that reach-out-and-touch-someone capability in Area Air Defence that the Frigates only have a Self Defence capability of, however, the Frigates do have the Harpoon over the horizon anti-surface capability... So we blur the lines a bit here. The real difference with us is that the Destroyers are vastly more capable when it comes to Communications - that corner-stone upon which all missions depend. The number and types of radios may be forever in flux, but the interface with those assets is the advantage the Destroyers have in their Operations Room.
I have more years in Destroyers and I do months in our Frigates, and would sail a destroyer any day. Upgrading the capabilities of the Frigates boils down to some space issues, but more so to her reach, in that her smaller mast limits some of her expandability wrt communications capabilities.
As for the swapping of crews. We have already done this. Mind you it was back in the early 90's with the first Gulf War, and it was a Destroyer, and there were issues, but what do you expect from a first try. The British and Americans do something else with their submarine fleets, in that they have the Red Gold and Blue crews. Essentially 1 and a half crews per boat. This permits a half crew swap at some point in a ship's operational tour, with the crew coming home for leave, courses or other professional development, and all the other reasons we have to come home. The ship benefits in that there is always half of the crew conversant with the current machinery state. Not paper conversant like a full crew changeout would be, but actual on the plates, hands on familiar.
Cheers