Hello everybody

I'm new to revit mep.
I stumbled this strange problem when trying to edit air terminals which already placed on ducts.
When trying to change size or duplicate it's size revit gives me this message: "The family no longer matches the properties for the System. Disconnect the family from the System?"
Well i don't want to disconnect it from the system (what exactly is that system?!) as well as i dont want to disconnect it from the duct.
why it wont just changes?

The wierd thing is that if i'll place a *new* instence with the size i want determind beforehand - it works ok!

here is my problem on video: (really short)
https://www.youtube.com/watch?v=6YGOeCt1Fmg