The Welsh assembly shouldn't be able to do this! This is the problem with 'partial' devolution.
The 'right' thing is to 'expect' FE colleges etc to accept the predicted grade, not the actual, then the DC goes to college as planned, moves on.
IF they regrade this exam, all they're doing is making it all but impossible for next year's cohort to get that C when they're borderline. There's no way the exam board(s) will be caught out again!
And another issue, what will happen when an academically identical Welsh DC and an English one goes for the same place in college, esp in England? The college will have no legal option other than to take the Welsh regraded DC over the English, will they? THAT might cause tensions, esp as places will have been awarded and courses begun by now!
Incidentally, I think we're all to blame for this, in a round-about way. I think that as modern parents, with the forensic obsession we have with every nuance of our DC's education, the scanning of League Tables, the dissection of OFSTEDS- the temptation to show year-on-year 'improvement in our DC's academic attainment must have been irresistible, but inevitably, that house of cards had to tumble somewhere, and this year's cohort were the apparent victims, but, in our classic 'as long as I'm alright, jack' way, all we're doing is pushing this 'problem' to next year, when our own DC will have cleared that hurdle. I do not blame any parent in that position, but again, we, via the schools, shouldn't actually be 'allowed' to do it!