Replacing Drywall vs. Refinishing Drywall

[Matt Vaughn]

Guys, I have removed and replaced drywall for 2 of the 4 walls of a bathroom I'm working on (removed to install the tub) in Maryland.  The other two walls are in fine condition (need a patch or two only.  

If I sand the old walls, when I paint the whole room, will the old walls look different from the new drywall walls?  Would you recommend that I consider replacing drywall to provide a consistent finish?  

Thanks for the feedback.