We've got 4 lines on a grandfathered family plan, for most of the last almost 10 years we've been happy with T-Mobile but the T-Mobile network coverage has deteriorated so much in the past 2-3 years that I'm ready to switch to AT&T even if it means costing us more money (and it most assuredly will).
The only issue is where my wife works (in the hills) the only carriers that have coverage are T-Mobile and Verizon, this is actually the reason we moved from the pre-Cinglular AT&T Wireless to T-Mobile in the first place.
What I want to do is port the lines for me and both our children to AT&T and port my wife's line to GoSmart.
I wouldn't be doing this until the last of our lines (one of our sons) goes off contract in 2 days.
Right now my wife and my lines are the "primary" lines on the family plans and our children are on add-a-lines, not sure if that changes anything.