How to easily set a master ID without losing everything?

Hello. I have a particularly smart person who decided to switch out some masters for some older version. Long story short, I would like to set these masters’ IDs to be the same as the IDs of the masters in my old version.

I tried setting the master ID with:


but then, all my glyphs in the master were suddenly empty and all data was erased for the master. I’m sure I’m missing something, but what is it? Thanks!

The master id is used in all kinds of places. The easiest is to do it in the .glyphs file in a text editor.

Great, thank you!