bmbahia, thanks for asking that very good question on TCAS, I had never thought about that aspect before.
But I'd would like to disagree on the following :
That's why I change the AP to the other side before changing the altimeter. To avoid such clb/des, although it's so brief that you will only change 10 or 20ft
If you change the altimeter setting or reference once you're in ALT CRZ GREEN or ALT GREEN, the airplane won't climb or descent by itself.
FCOM 1.22.30
The altitude that ALT mode holds is the altitude it memorized when engaged. It is not affected by a change of barometric reference or by a change in the barometric correction