I do like your idea very much. The switchable rounds will make SU76 more viable in most situations. Make it 45 range will be ideal. It is shorter than all ATGs range and a bit longer than most man-portable AT weapons'. Round changing time could be anywhere between 3 to 5 seconds. Long enough for 1.5 ATG shots will be excellent.
If the balance team considers this, they should also consider toning down its mobility a tiny bit to prevent su76 hordes roaming around the battlefield. With slightly lower mobility they will be more susceptible to AT guns showing up nearby and tank flanks.
I really want this to happen because then the Soviet T3 will finally have 3 must build units.
T70 the workhorse of the midgame and later the best scout unit.
M5 AA quad flank defense and mobile suppression as well as excellent AA.
SU76 infantry support, soft anti tank and later with vet could support vs heavier armour.