Depends some on how out of date your previous Trellis version was. 
In my experience if your server is already provisioned then you can still deploy without reprovisioning. Of course, if you want any updates that a new Trellis provides (i.e. that PHP version bump) you’ll need to reprovision. If you do upgrade PHP, keep in mind that you may have to manually restart the PHP service. Usually I don’t bother reprovision a remote with updated Trellis, unless I’ve merged in Trellis changes to add a feature I need it fix a bug/plug a security hole. I updated Trellis and ran deploys without reprovisioning on 4-5 sites last week, and they all handled it fine.
Of course, backups are always a good idea when reprovisioning. On Digital Ocean I usually take a live snapshot before I run a reprovision.