Well, if you're asking in the middle of a migration, be careful wtv you do!
Otherwise, disable NR reg will kick them off the core cm. If that were remote branch 3, in NR3 with a gateway and LSP, all disabling reg on NR3 would do is kick NR3 into LSP mode.
Depending on your architecture and how you're trying to migrate, making those NRs disconnected from all dsps in 5.2 will guarantee a reboot provided no GWs are active in LSP mode.
Or, you can probably feed up a station import with ASA to change the security codes of a bunch of phones in 5.2, reset ip-phones and specify a subnet or extn range to force a reboot, new 46xx to point em to CM7 and do it that way.
Couldn't tell ya what makes most sense for you, but mixing and matching a few strategies should get you out of the woods.
Are you slowly migrating, like just the sales Dept in site 1 on voice vlan 1 need to go to 7 but everyone else there stays on 5.2?