The Sibelius Forum

So... I have a whole arrangement in 4/4, but the feel is ternary, and some bits just cry for the meter to be in 12/4 but writing the score from scratch would s*ck!

Select all the bars you feel are in 12/4, but make sure the total number of beats is divisible by 12. Press t and enter the new time signature. If you have selected it correctly, that should insert the 12/4 ts and restore the 4/4 at the end automatically.

But the music is in crotchets, it won't simply turn to dotted crotchets

If that's what you want, are you sure you are describing the issue? It sounds more like you are aiming for 12/8, not 12/4.

There is a plugin called "convert simple time to compound time" that might be able to help. It's a shipping plugin, so it should be available to you. It makes a copy of the selection immediately after it., so maybe use a new score temporarily.