Is the division clean? Meaning is there a good split between buildings in the campus? If there is, then that's easier, but if there's not, then it'll take some work.
1) rename campuses to be identifiably different (i.e. master vs secondary)
2) go to VRF Setup in AirWave -> toggle HTML5 off, save -> aim is to get into Flash mode since the feature has not yet been ported into HTML5 code
3) go into the secondary campus, right click on a building -> change campus to the master, save
4) repeat for all buildings
If you have buildings that are split, you'll just have to go 1 additional layer to rename the secondary buildings, I usually append an extra number/letter at the end of the name, so building1 = building1a to differentiate from the building that's on the master campus.
Then instead of right clicking on the building, you'll drill into the building view and right click on the floor. And then from there select campus & building to migrate.
Once all this is completed, go back to VRF setup and toggle back to HTML5.