How to keep the same tempo after time-signature change?

I'm new to Musescore and would like to know whether there is a function similar to the conventional tempo instruction 'crotchet = dotted crotchet' at a time-signature change.

I'm transcribing a guitar arrangement of Bach's 'Jesu, joy of man's desiring' in which the time signature switches between 9/8 and 3/4 but the three-beats-in-a-bar pulse remains constant. At the moment the playback is keeping the same duration for a quaver in both time signatures, so that the tempo increases whenever the music goes into 3/4 (because a quaver in compound time represents a smaller division of the beat). I don't want to write the whole thing in 3/4 with triplets or 9/8 with ties. I suppose I can figure out the relative bpm and enter tempo text at the appropriate places, but if there is a better way then I'd like to learn it. Thanks for any help.