Hmm... well, it's not as simple as an 'undo' button, no.
Assuming you don't have a backup of the npc_types table to restore from, you could create a new npc_types table with the correct npc_types.class values from a default peq install.
So, for example:
npc_types table. This is where all your classes are 71.
npc_types2 table. This is a new table you have sourced, where class values are correct.
Then, do something like...
Code:
update npc_types npt
join npc_types2 npt2 on (npt.id = npt2.id)
set npt.class = npt2.class
This is my first thought at least. Others may have different suggestions. Also, this wouldn't update any npc ids you have created which do not exist in the 'new' table.
Good luck.