Will I lose the customizations? Will my game revert back to the Maxis neighborhoods?
I generally move my custom neighbourhoods out and back them up before installing a new EP (well, I back up all the neighbourhoods). After installing the EP, if you move your neighbourhoods folders back into the Sims2\Neighborhoods folder in the My Documents section, and move your CC clothes, objects, etc. back into the Downloads folder, your sims should not lose any customizations. The game will update your 'hoods to be compatible with BV (save your old backups - the BV versions of your 'hoods will not be compatible with earlier EPs, if you decide to ever unload BV).
You should make sure that your CC objects are compatible with BV, and get the latest director's cut of Awesome hacks. Incompatible hacks are a major source of problems for a lot of people. As some modders have dropped out, some of your current hacks may not be available for BV (if you use a lot of non-awesome hacks, anyway). There are discussion threads on this here if you search using the name of the hack.