This is a little bit difficult, because the players are save with the old values for the skills. So the process involves two steps, one of which can be done either through code or by simply searching and replacing in the save files.
You need to "bump" the saved skills in the save files - for instance, a save file might contain something like this:
Code:
Skil:
131 40
132 45
139 21
0 0
Let's say you want to put MAX_SPELLS at 10130 (for simplicitys sake), so you want to transform rows with numbers above 130 to the same with 10 added in front. This is perhaps doable by hand, depending on how many players you have, or chatgpt or similar can help to write a command line utility for it. This is of course also possible to do in the C code, but I think it would be easier with a script.
Once you've got a script for it, stop the mud, make a backup of the player files, run the script, adjust MAX_SPELLS and MAX_SKILLS (and the numbers on the actual skills in structs.h) accordingly, and reboot. Now, everyone should be able to log in with correct, higher numbers, and everything should just work.
So, it's a litte bit of work, but you can test your script on a copy of some player files, to make sure you only alter the lines that need altering.