I have an electrical connector in an assembly and it (the connector) is fully defined. Let's call it part "X".
Now, as I look at the portrayal of part "X", I realize (even though it isn't necessary in this particular assembly) that if this connector had the appropriate strain relief in place, it would look much better as a visual than just the connector itself being shown.
So I create a simple 2 part assembly with the connector (part "X") and the strain relief (let's call it part "Y"). Now, going back to the big assembly, I find the connector (part "X") in the feature tree and do the "replace component" function, choose the "XY" subassembly and swap.
What I can't understand is the every mate that was put on part "X" is now lost, even though part "X" is within the subassembly. Maybe I should have brought the strain relief into the main assembly, defined it in place and then created the "subassembly" from there.
I see the "reattach mates" check box but this doesn't seem to do the "complete" set of mates applied.
The mates are looking for 'face@partX' or some such nomenclature. However, because part X is now part of subassembly Z, the face would now be something like 'face@subassemblyZ/partX', so SOLIDWORKS doesn't recognize it.
If you shift select the broken mates in the tree, then select edit, you'll be able to easily repair the mates.