Just some advice, some probably know and some may not.. but if you imagine your chords and how you play them when in G tuning. Imagine D position in a song in G, in G tuning. D is the V chord.
Now put your banjo in D tuning, and play the D position one set of strings up... in other words, instead of playing the 3rd string 2nd fret, 2nd string 3rd fret, and 1st string 4th fret, just shift up a set of strings to 4th string 2nd fret, 3rd string 3rd fret, and 2nd string 4th fret. And you're now also playing the V chord, A, to D in D tuning.
See how that works? You may have to get the banjo out to see it work.
By the way, this works for all chord changes.