Watch the Banjo Emulation online guitar lesson by Joe Dalton from 100 Twang Thangs
This is a great way to back up a section of music or even to use as part of a solo. It will make things more interesting for you as the player as well. Throwing in the 9th chords where you had 7ths should do nothing more than add flavor. These dominant chords are for the most part interchangeable. Try them and see if they work. Use your ear and your judgment. If it sounds good use it.