I was wondering if I could replace just the neck of a 6 string acoustic to a 12 string. I've been playing bass for a number of years, so my hands feel cramped on a normal 6 string. Even tried to learn lefthanded to no avail.
Assuming it's a bolt on, then yes, you could. However, finding a neck that fits the pocket won't be easy,and you'll be left with a spare neck and possibly a Frankenstein guitar, so your best bet will be to purchase a 12 string - check its fingerboard IS actually as wide as you need - and take off the six strings you don't need. You're going to find the remaining strings a little lop-sided on the neck, though.
I am not aware of a guitar with an acoustic neck that is interchangeable between 6 and 12 strings. 12 string guitar necks are beefier overall to handle the greater tension as well as make room for the additional strings. The neck pocket on any bolt on design for a 6 string would not be wide enough to accommodate a 12 string neck. Of course a qualified luthier could make you a wider neck that might work or modify a 12 string neck to bolt on to a 6 string body but that will get expensive.
Try a classical guitar which will have a much wider neck. Or if you insist on a steel string, some guitar manufactures (such as Taylor Guitars) offer wider necks when you order the guitar. For example any Taylor Acoustic 300 series and up comes standard with a wider 1 3/4 inch neck (as measured at the nut) but you can order one with the more common 1 11/16" neck or an even wider 1 7/8" neck (as wide as many 12 string and classical guitars).
Other manufacturers offer the ability to order guitars with wider necks as well and the standard neck widths vary from model to model so if you look at the specs you may find some that come standard with wider necks.