If 100 senior wide body pilots displace 100 narrow body pilots, what training does this trigger for narrow body pilots? Aren't they out the door? The only reason 100 senior wide body pilots get to displace narrow body pilots is if there is redundancy. Therefore your math is wrong.
If 100 senior wide body pilots displace 100 narrow body pilots and there is redundancy, some junior narrow body captains will have to be displaced to become an FO, because there will be too many narrow body captains plus many of the junior narrow body FO's will be made redundant and need to be replaced. This is the training. This my maths.
If you retrench the 100 most junior FO's, this means 100 junior captains will have to be retrained to be an FO again.