Personally, I would proceed in 2 stages (more balance with time and cash flow):
1) continue with the same lower, but modified bolt and 7.62 barrel to please ar mag crowd.
2) Since upper is done in stage 1 and some rnd costs are started to recover, modify lower for ak mag crowd.
End result: 2 uppers and 2 lowers and potentially 4 kinds of guns to sell: 223 ar mag, 223 ak mag, 7.62 ar mag, 7.62 ak mag.