Using Calibre, my various ebooks formats have been converted to epub with PAGE SET UP/OUTPUT PROFILE set to: sony reader 300. If I buy a new reader, will I have to reformat my books to match new reader profile size?

I would expect they would largely work without any issues as ebooks are designed to text-flow according to both screen size and current font size.

The only time knowing the screen size matters is when one is trying to output graphics that are meant to fill the screen. Therefore you might want to re-convert comics, although even with them they should be OK if you do not mind not using all the screen space.