Single Shower Faucet Replacement....

Somebody, Somewhere, Someday! Please tell me if it's impossible to take off the shower faucet and replace a new one? Does it require tearing tile out of the shower??? If I am just replacing the faucet itself, wouldn't my building supply store have a universal unit to install back? What if I wanted to replace the shower head as well? Do I have to tear out the tile to replace it???? The reason I am asking is because I have Valley faucets throughout the home and it's hard to find replacement knobs. The knobs are broken off most of them, INCLUDING the shower. I've been though a half dozen of their handles and they aren't holding up too well. I want to change out the fixtures on all faucets on the lavatories and the showers as well. Please tell me it can be done....
ThXX in advance to anybody that can help me...

You already know the bad news about Valley. Handles and stems and cartidges and seats and springs are still readily available. That is all that is available to you. Changing the valve requires access inside the wall, possibly from the other side through a closet or other wall.

Lav faucets are easily replaced.

Changing the valve requires access inside the wall, possibly from the other side through a closet or other wall.
I didn't want to hear this...so its tear out the board and tile to get to the pipes to change the valve or tear a hole on the opposite side of the wall to get to the valve easier? GREAT! So the valve is the real problem. So I may as well call a plumber instead of having my "DIY husband" get involved in this one....I appreciate your help...Thxx

Generally speaking, you have to stick with the same brand when you replace fixtures and stems, etc.. The shower head is different. Just unscrew the old one, add a little teflon tape to the shower arm, and screw in whatever shower head you want.