Depends on whose account will be kept.
Your account will be merged with your fiancee's account (hers will remain open):
- The channels package(s) that she subscribes to and discount(s) that she has will be the same. Unless of course both of you are in agreement to change the channel package(s) to the one that has the channels that you want.
- Any discount(s) that you have on your account will be permanently lost upon disconnection.
- Her monthly bill will incur an additional $6 fee and monthly billed amount will change based on the subscribed channel package(s)
- Any grandfathered or expired channel packages(s) on your account will be permanently lost and not transferable to her account
- You can activate or move all your equipments on her account (with her permission, of course, since the monthly bill will be affected)
Her account will be merged with your's (her account will be closed and yours open):
- Any ECF/ETF on her account will be waived if merged with yours
- Any discounts on her account will be permanently lost and not transferable to your account upon her account disconnection
- She can activate or move all her equipment on your account (with your permission, of course, since the monthly bill will be affected)
For Installations, as @peds48 mentioned, it will depend if the existing system will have enough tuners available. If you are confident enough to do the installation yourself, then you can do it yourself. Otherwise, DirecTV will have to send a technician-generally with a fee for the work order.
I think YouTube has some tutorials; have seen it floating around there.