As
Trenloe suggests - having ASC/DESC as an option is sooo much more complicated than you could imagine. I personally hate ASC AC but if you are not going to have ASC AC then CnC is already a better ft straight away.
Trenloe also has it exactly right (at least as far as what my thinking was) in regards to using the CnC as a base.
For AC on Charcater Sheets - Characters are all new (you are not importing CnC characters into your game so you would
replace AC with the DESC version and you would replace BtH with THAC0.
For NPCs howver we have a whole library of existing critters coutesy of Monsters & Treasure (included in CnC) that already have ASC AC and a BtH value. For these you need to
calculate or replace these with the required AD&D values. You would probably need some mechanism to check if AC already exists - if so convert and store as ACDnD. If it doesnt exist just use the ACDnD value. BtH for monsters is dependent on their HD in CnC and (I cant remember for sure) THAC0 probably is too.
Of course this is an abslutely simplified overview of the tasks - it is still a significant task.
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You couldnt just rip all the code out of CnC and make your own ruleset based on that and distribute as a community release as that ruleset has been released commercially. You could build on it though and it would (at least from where Im standing) be a much smaller job than building from scratch.