I'm trying to notate a song which starts out in 4/4 time (with two 8th notes per beat) and changes to a triplet feel (with three 8th notes per beat and four beats in a bar) part-way through the song. Currently I'm notating this as a change to 12/8 time, but my tempo is given in quarter notes per minute meaning that after the time signature change the tempo effectively slows down (because there is now 1.5 quarter notes in each beat rather than 1). Is this the correct way to notate a change in feel, and if so how do I get around the tempo problem?
It depends a bit on your notation programme whether or not playback will work properly, but, from a notation point of view, you either notate triplets if the change is fairly temporary, or you use something like
over the bar line that demarcates the change if the change is going to stick around for a long enough time (which is your judgement call). The alternative is to use "dotted crotchet = <existing tempo>" over the first bar in the new metre.
Simplest way is to annotate at the beginning of the 'swung' part with a crotchet = crotchet and quaver, with a slur under the latter two. Even with the word 'swing'. Such as you'd find at the beginning of a whole piece that had a 12/8 feel, but was written in 4/4.
If you want to keep the same quarter-note beat you could write triplets. If this becomes unwieldy, you could write 12/8. Yes, a sequencer will need a tempo adjustment. Maybe you can tell it directly 'dotted quarter=120' (or whatever) instead of 'quarter=120'. Or it may recognize a 'metric modulation' notation. The score publishing programs (Sibelius, Finale...) can cope with this. Ones that aren't notation-based may want a literal 'quarter=120' even if the beat IS a dotted quarter. You'll have to do the math.