The Orion MIB database is generally added to, rather than deleted from, so if you have devices that are supported currently, they should remain supported after the upgrade.
If you're asking if your Custom Pollers, or Universal Device Pollers, are kept post upgrade, then yes, they will still be there.
Having said that:
ALWAYS BACK UP YOUR ORION DATABASE AND ENSURE YOUR PLATFORM SERVERS HAVE GOOD RESTORE POINTS, BEFORE INSTALLING ANY ORION UPGRADES!
Always be prepared for the worse, it will save you oodles of time in the event that an upgrade epic fail