The "master copy" would be at conception. Even after a few divisions it is unlikely any of the cells carry the exact same DNA.

And just because you made it from zygote to birth does not mean your master copy was "pristine".

But yeah, if you could somehow replace your tissue stem cells in situ with ones that are closer to the original state it would generally allow your body to continue functioning for much longer. I mean some of those mutations could be beneficial to your particular environment too, but in general... Yeah.