Divide up each PCs XP total into pairs of numbers and read them as Humanoid (or Animal, or Plant) Mutations on the Gamma World table of choice and explain that the world has entered a State of Mutability of unknown duration. These mutations are affecting everything in their world and new monsters and powerful enemies are on the move.
Everything now has base CONd6 Hit Points, and Class Based HPs are just a kicker which are consumed first, except when struck with a Critical Effect which wither the CONd6 HP. These meat points are much slower to heal, 01 Point per Week of Bed Rest +/- Con Modifier; yes it is possible to get worse, not better.
Any Class Features are frozen at present Level, as Level no longer affects development, but only applies to Reknown and Allegiance Status. Use the GW version of Development rules (1e is pretty random, 2e on is less so).
Use every creature you had before, but add the CONd6 HP, and give them 2 or 3 Mental AND Physical Mutations. Add in the GW creatures you like, hell, add in the ones you hate and place a bounty on them.
I guarantee that the combination of Magic, Mutations, and High Tech will revitalise the game for everyone.