ok ... i think is better when you install a fresh version of OMV 3
True, but after what seems to be a successful in place upgrade on my DR box, I went ahead and did the same on my production box. Removed all plug-ins, did all available updates, then did the 3.0 upgrade. It went very smooth on the production box and now both seem to be running v3 just fine, and I did not have to re-do my entire configuration on either.
Had the DR box blown up and needed a clean install, I would have just gone that route for both to be safe.