Jay,
Evidently it can be done but I'll leave it to one of the experts to let you know exactly what needs replacing.
Don't give up on the 20000 as a low geared popping reel, you may find it works OK. The low gear works fine when using a popper and there is more than one experienced GT fisherman that also uses them for stickbaits. Sometimes slow is good.
Regards,
Trev