How to make a small edit without ruining the whole glpyh

I want to shorten the middle leg of the letter m. The generated font file doesn't show the edited character. I uploaded the font to an online analyzer which shows the modified glyph.

But preview text does not use the modified glyph.

If I move the middle line horizontally FontForge gives me this message and the generated font uses the shortened middle leg.

You have just changed the point numbering of glyph m. Instructions in this glyph (or one that refers to it) have been lost.

But the problem is the whole m is taller and skinnier.

How can I shorten the middle leg without deforming the whole character?