I'm trying to add a section on converting from the descending AC used in older editions to the ascending AC used in Microlite74 for the section on "0e Conventions" section in the Microlite74 rules. Here's what I've come up with. It's easy to do the conversion, but I seem to be having trouble writing it in a concise, clear way. Here's what I have. Suggested improvements are welcome. I've been working on this for 24 hours and only making it worse.
Descending Armor Class: 0e and other pre SRD editions used a descending Armor Class system where an unarmored character was AC 9 (AC 10 in some editions) and better armor used lower numbers (e.g. AC 5 was Chain, AC 2 was plate). Magic armor could even have a negative armor class. Microlite74 uses the ascending Armor Class system used in the OGL SRD. If have old adventures using the original descending AC system and wish to use them, it is easy to convert descending ACs to ascending ACs.
Unarmored AC is 9: If the adventure is for 0e (or other edition where the unarmored AC is 9), simply subtract the descending AC listed in the adventure from 19 to get the ascending AC used by Microlite74.
Unarmored AC is 10: If the adventure is for 1e or 2e (where the unarmored AC is 10), subtract the descending AC listed in the adventure from 20 to obtain the ascending AC used by Microlite74