You will never get a M2 to 500$ Carrack from buyback unfortunately. It is not possible, because even if the Carrack would be at 500 in that CCU, the M2 would have jump to 520, so invalid CCU.
Your only chance is to have someone with that CCU in hangar.
Or as you ask, a 600i explo from buyback would still be valid for a carrack at 500$.
When there is a price jump, the TO ship price does not change 99% of the time in BB, when the FROM ship always see its price changed when released (M2 480 to Carrack 500 would now be in BB a M2 520 to Carrack 500).
More info in my guide about upgrading in my signature if you want.