Fortunately we know the ROF 57mm 6 pounder can be changed to 75mm on new guns as this is what happened in OTL. Both so that they could use the same ammunition as US tanks and also to get better HE which is what tanks normally need to use.
Quite.
So, what was the situation RE these weapons having degraded armour penetrating ability with their new calibre?
We were discussing maximising AT gun firepower...