If nothing else, though, if you draw enough new strips to fill out a new book, you can always put in the older strips drawn in the smaller format in the book, just more tightly layered with like several on one page or something like that, as bonus material at the beginning or the end of the book.

Its sounds scarier than it really is.  
Digital zoom can make everything seem small or at the very least the same exact size as what you're used to working with, and the only time you end up noticing the size is either A when you zoom in and notice no real decrease in quality, or B when your computer starts taking longer to do everything than you're used to.

Have faith in yourself!