I'm migrating a customer from a Stratagy Flash to a LVMU. Is it OK to do a back up and restore from the Flash to the LVMU? If anything there mostly concerned about saving the lengthy, English and Spanish greetings stored in mailboxes 990, 920 and 940.

I don't recommend simply because the old flash is clearly not the same version as your LVMU

No it is not possible!

You will have to reconfigure the LVMU from scratch