Depends how much you have done to the SQL database and what Unity will think of it.
Regarding Failover, remember, the Failover server is replicating the entire database from the Primary to the Failover server. So anything you have done in the primary, is on the failover. If table were altered, there is a chance that it was not replicated. But most likely it was.
i would set up a lab enviroment. Load a server with 4.2(1) on it. Run a DiRT backup on the exisiting enviroment, and the run DiRT restore in your lab and see how it goes. Test all the pins, greetings, recorded names, leave a message, MWI on, MWI off, etc.
Worst case, you can run DiRT or ExMerge to export all the voicemails out of the server (if you are using voicemail only setup). Install a new server and ExMerge them back in. Users will have to set up their recorded name, greeting and new TUI password.
Take your powers that be and slap their hands for altering the tables! ha
good luck