Lon,

you're talking about a difference of 300dpi vs 292.5dpi at the print stage. Yes, you will have a degraded image, but will you be able to see it? Only if you know what to look for and with a loupe!

BTW, I work the way Joanna described. Scan once, create one master file at scan size and resize the master file. Takes a little longer to work on, but saves a lot of time later.

Congrats on needing to print a 20x24!