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Hello and thank you for your question.
Before you cut everything apart, consider this:
Shut the water off, drain it down the best as possible. disconnect the toilet supply tube from the valve to the toilet. then try placing a wrench or pliers on the valve and turning it to the left to loosen it. IT APPEARS in the pictures that this is a 3/8" chrome threaded nipple into the valve, although it is so corroded it is difficult to see if there is actually threads there or not. It is certainly worth investigating before cutting the wall open!
Additionally, if the tube begins to twist and not un-thread, stop. At this point, you will need to open the wall and replace within.
I truly believe this is a 3/8" threaded nipple as the toilet supply tube appears to be 1/2OD which was used back in the 60's and 70's. This would be before they came out with soldered supply valves like this. If this is the case, as I believe it is, then you can simply unthread and replace with a new 3/8" chrome nipple and 3/8" IPS by 3/8" compression toilet supply valve with a standard 3/8" x 12" toilet supply tube.
Sounds good. Let me know!