I see nothing in the FCC regulations that force one to change frequency. Use of their repeater, OTOH, may not be allowed.
Note that the FCC considers those grandfathered licenses to still be GMRS -- not some business (LMR) exclusive band. Making use of their repeater, OTOH, may require asking permission (and likely not getting it).
The primary facet is that the only modification to the license allowed is with regards to contact information. They are otherwise stuck with the frequency (pair if repeater), antenna height, location, power, etc. as when originally licensed. The City of Grand Rapids runs a repeater on .575, 50W transmitter with 200W ERP on a 290 foot tower (or maybe building roof). On a "good" day, that repeater can break squelch all the way out to Lowell (it's supposed to have a 35 mile radius). The 1997 PRSG Repeater Guide listed 141.3 146.2 as available for EMERGENCY/TRAVEL use only (any other usage had to get prior permission from the city -- and likely a different CTCSS tone).
What does the city use it for? Especially since the whole state has converted to a massive trunked digital system? Well, while undergoing chemo sessions, I get good signal of the staff running the city parking lots having troubles with gates not going up/down, payment machines not accepting cards, etc.