Can I change a 1-gang electrical box to a 2-gang electrical box?
As long as you keep your current consumption under the rating for the circuit, that's completely OK.  Getting the old boxes out will be tricky, but can be done without too much damage to the surrounding drywall; if the old box is mounted to a stud, it may be easier for you to cut the nails than trying to pull them out.  You'll probably have to use a remodeling box anyway, so the old nails won't get in the way.  

Depending on your local codes, you may or may not be allowed to do the work yourself -- check with your city (or whatever authority applies) to be sure.