What do you mean by " reclassifying the next $6k above that as catch-up since he's over 50."? Does he make more than $270,000? The 6% should come out of his pay until he hits $24,000 or the end of the year.
If he made $400,000, the $24,000 is 6% of his pay.
Don't confuse it with the % used for the ADP test. That would be 6.67% ($18,000/$270,000).
You do not look at the comp limit when you are implementing the participants deferral elections. Just plan limits and 402g/catch-up.
Deferrals are not "reclassified" until they go over one of the limits: 402(g), ADP test, plan limit.