I was taught to change the key at the next measure, and add accidentals to the lead-in notes. You can change time signatures and clefs mid-measure, but I'm not sure about keys.

If you must do it that way, change the time signatures of the two measures to 3/4 and 1/4, with the time signature set to display as your original 4/4. You can leave the double barline, or make it invisible.

Please add the program and version you are using to your signature, in the user control panel.

I actually found a great way to do!: If you do a mid measure repeat then change the key, you can then just delete the repeat and the signature stays!

… If you do a mid measure repeat then change the key, you can then just delete the repeat and the signature stays …

It is really the same as the solution Mike Rosen suggests: use two measures (of 3/4 and 1/4)

Instead of using the plug-in Mid Measure Repeats you can also use the plug-in Split Measure.

A plug-in only does what you can do yourself, “manually”.

The plug-in just does it faster.

I encountered a similar challenge and resolved it by the 3/4 + 1/4 technique.

I also needed the double bar remaining.