Channels 1-4 are simulcast on the 700 overlay and Zone 1 trunked systems. There are also some BFD talk groups on the 700/800 that are not simulcast on the UHF system those can be found here: Boston City Services - Scan New England Wiki
My understanding is that there are currently no plans to leave the UHF. Like other T band users the longer range future will depend on what happens with the T Band changes that have been put in place by congress.
It's as Gordon says. There will be increased use of the trunk system for non operational traffic, but the UHF-T channels are here to stay.
It's likely the Congress will take another look at D Block in the next session. There is a lot of push back from large cities that are heavily invested in T Band systems and don't have the money to move to 700. That's a different story, though.