The Excelsor's nowhere near as maneuverable as the ships that are maneuverable ... like Birds of Prey and what not.
I guess you missed the sarcasm smilie. The Excelsior is already as manouverable as a Qin Raptor. If you ask me, the Excelsior should have never come into beeing as a T5 ship. Over hundret years old and better than all those new shiny cruisers... :p
Tried and true, modular design. They probably had a ****ton of them which are just as easy to upgrade as it is to build entirely new ships, just like the Miranda.
Personally? If I was a starfleet admiral I'd never retire a ship until it was falling apart. I mean, if I retired a class, that means that no more are being actively built, or it's role is being filled by a newer ship. No point in sending a bunch of ships that are in perfectly good condition to sit in some surplus yard.
Those old Akula class ships (starfleet command)? I'd use them as convoy escorts or planetary defense if they aren't easily upgraded.
You know its not a bad idea. It's similar to the light cruiser retrotfit that was thrown around a long while ago. You sacrifice several weapons slots and some hull strength for a goodly return in maneuverability.