Three different ways for dulcimer player to make relatively quick changes from D to G: (1) If you've got two or more dulcimers, string one with light-gauge strings -- I use two 9 or 10 gauge and one 12-13 gauge. Tune the heaviest one to G, others to D and you've got yourself a more-treble-than usual dulcimer tuned GDD or G Ionian. To retune this one for your DAD tunes, just tighten the heaviest string up to A. (2) Use a capo. A chopstick or pencil and a rubber band will do; commercial models are available. Whether you started in DAA or DAD, capo at third fret and you'll be in G major. (experiment with capo at 1st fret for E minor and fourth fret for A). (3) from DAD, tune your middle string down to G and you'll shift from Mixolydian D to Ionian G.
Sorcha, I'd like to learn the tunes for Cat, Farm and McPhersons, but don't know if I can read abc. Any chance you could type them down by fret numbers?