I have a microSD card with dietpi mounted on a raspberrypi 2B . If I put this card on another rpi3 could I have problems at boot? I only do this because the rpi3 is more performant and recent.
Basically you could create a backup and just give it a try.
Yes, of course. I already have backup copies and lists of installed packages. If this gives me problems I would start with a clean installation on a new microSD and then update the package list. I was interested to know if anyone had experience with this. Since a database and vpn server runs on the rpi2 I was interested to know if I could put the card on the rpi3 without any problems. I’ll try it today and let you know 
Personally, I tested the switch from 3B+ to 4B without issues. Never did that from a Pi 2.
OK, thank you very much. I will try today.