When switching keys with simple diatonic instruments like tabor pipes, it is more helpful to think in terms of where the tune goes in the diatonic scale, rather than what the note letters are. You can do this intuitively, as when you learn a tune by ear and can play it on whatever instrument you have in front of you, or you can use a movable-doh
Solfège to assign names to each scale degree: doh (the tonic), re, me, fa, sol, la, ti, doh. Your sheet music may have a tune in C with notes C, C, G, G, A, A, G. To play that on a whistle other than in C, think of the tune as doh, doh, sol, sol, la, la, sol, or XXXXXX, XXXXXX, XXOOOO, XXOOOO, XOOOOO, XOOOOO, XXOOOO. Play that on any whistle, and you'll hear the opening notes of "Twinkle, Twinkle," without having to keep track of what note letters you are playing. (On a tabor pipe, you could notate it something like XXX, XXX, XXX', XXX', XXO', XXO', XXX'.)
Others have explained it better elsewhere on these forums, but I can't find their posts just now.