Would I have any problem upgrading my BB to an EVO but then letting my wife use the phone with her sim card? I'm the primary account hold and I have heard it can take 3 months to transfer this to another number on the account.
How about you tell us what you want to do and how you think you can do it, and we'll tell you if you're right on or a better way to achieve what you want?
It sounds like you want to upgrade to the EVO, but give it to your wife, and you keep the BB. You have to wait at least 30 days to do this. An alternative is to ask Sprint to switch the primary account status to your wife's number. This can take 1 - 2 billing cycles, depending on how close you are to the end of your cycle. This way you get the phone on the right account right off the bat.