My major obstacle would be re-rendering at 300ppi.  I'm going to have to re-read that other thread on the 300ppi to see if I understand this whole "render to file" stuff. 

Otherwise, would I have to recalculate the size via the "layout setup" options?  I did that once for an 11x17 image once.  My computer took about a week to render it, and the file is over a gig in size. :c(  I can't afford that much space at this time for any image.