Tub fixture change Noobie here

Okay well im trying to help my mother remodel her mountain house and im stuck. Im trying to remove the old shower set in her bathroom and I got the old set off. The "gate" (I guess is the name? The part where the hot, cold, shower and faucet pipes screw into. 4-way thingy) piece that came with the new set is a different size than the one that is existing. OF COURSE IT WOULDNT BE EASY.

So my question is this how can I get the old one out from behind the tub and install the new one without tearing up the drywall??!! The tub is in the basement and there is a slab on the wall behind the faucet. The builder made 4 inches of room behind the drain side of the tub. Help!!

Thanks in advance and great site!!!

They make remodel plates that would cover the bigger hole you'd need to replace this from the tub side. That's probably the only way. Last week, This Old House did exactly this...you might still catch that show or catch it on-line to see exactly what they did in a video.

faucet

The easy answer is that you may not be able to, depending on how the faucet is installed in the wall, and what the new one looks like.