To create imprints in my piece, I created an assembly in which I insert imprints.
The problem is that the part and the assembly were created under SW 2012 and since the transition to SW 2015, to modify the footprint SW tells me that this footprint is linked to another assembly that I don't know!
Is there a way to cut the link between my assembly and this phantom assembly?
It may be because you have registered your assembly by having the link of the part left with the original assembly registered.
To redirect your part link to the new assembly:
From the Explorer in SolidWorks, go to the part, click on Reference, then choose the new assembly, then open and saved, this re-routes the link of your part to the new assembly.