Hi Laura. Not so much a key change as a move from Major to Minor. I do it in A minor,

(Am)Its a rosebud in (Em) June and the (Am) Violet's in full (Em)bloom. The (Am) small birds are (C) singing love (D)songs from each (E)spray. We'll (C)pipe and we'll (G)sing love, We'll (Am)dance in a (Em)ring Love when each (Am)lad takes his (Em)lass all (Am) on the(G)green (AM)grass. And its (Am-Em)all to (F-G)plough where the (Am)fat oxen(F)graze (E)low. And the (Am)lads and the (Em)lassies do (F)sheep (G)shearing (Am)go.

You can put G to F as passing chords between the Am and Em each time it occurs. (Keyboard, box and harp players are good at that). You can also end with a final Amajor instead of an Aminor for a final cadence.

Sod the vocal importantance. The lines say "pipe and sing" and "Dance in a ring" which certainly suggests instrumentation to me if only a tabor pipe and drum. It goes well up to dance speed without all those dreary a capella pauses too. In 3 time syncopating the lyrics across the bar-lines like Carthy and June Tabor do. This is a fertility song of abundant harvest and should be sung with joy and fertile vigour. Forget the funeral dirge version by Steeleye everyone copies. Read the words, sing and play it your way and "make a fine show"