One technique is to treat the 7A value as a base36 number. Convert the base36 7A value to a 64-bit integer (20I 0), add 1, then convert back to base36. I haven't had a need for this, so I don't have a working example for you, but they're easy enough to find. Search for "convert base36 to base10 rpgle" (or without the rpgle, if you like).
There was a recent article here: TechTip: Compacting a Large Number into a Small Space | RPG. It has routines (see the link for the .zip with the code) that do the conversion to and from base36. (I haven't looked at their code.)
One caution - you may not want to use their code out of the box. In base36, the sequence is 0..9A..Z, which is standard symbols for bases greater than 10. (An example closer to home would be base16, which is 0..9A..F.) If you use this code directly, your sequencing would be '0000001', ..., '0000009', '000000A', ..., '000000Z', '0000010', and so on. That is, in each base36 digit, 9 + 1 = A, and Z + 1 = 0 with a carry to the next higher digit. If you want your digits to increment from Z + 1 = 0, and 9 + 1 = A with carry, you just need to change the base36 "alphabet".
In any case, overflow occurrs at 36**8-1. With standard base36 alphabet, 'ZZZZZZZ' + 1 = '10000000', which is 8 digits. With alternate alphabet, A-Z0,9, max value would be '9999999' + 1 = 'BAAAAAAA'. Either way you'll want to check for the max value before you convert.