Trigger Process on Changed Field?

I want to trigger a process based on a change of the Assigned To user. But Sugar does only allow me to check for a user's relationship within a given object. 

BUT I just want to see if the field has changed, not to whom it changed but only that it changed.

Later on, that new Assigned To shall do an approval. 

It seems Sugar is unable to allow this simple requirement, or am I just blind?

(using only Sugar 8.0.1 without any addons. Just the plain Process Definitions module)