You are certainly altering the club BUT you are not necessarily making it non-conforming.
As I indicated, your club may well conform and is therefore perfectly legal for play. However, if you are challenged and need to to prove its conformity, then you would have to get confirmation from the R&A/USGA.
If you are not challenged there is no problem
Agreed and this is what I've eluded too.
We had a new country member afew years ago that I got to play with on several occasions when he first joined ( a solid 4 marker )
He had made his own putter from a 4" long section of 2" round gal pipe, a piece of flat 1/4" stainless steel 4" x 1.5" welded to the pipe, drilled a hole in the top and glued in an old iron shaft with a normal grip.
Because I'd played with him abit, some of the older rules inclined members had asked me to point out to him that his putter was non conforming. I spoke to him about the home made putter but not because it didn't conform but because it hadn't been approved !
He took it very well and the next week turned up with a brand new $1200 Scotty Cameron circle T and then proceeded to street the field and win a big board event.