I understand that mates that are made between parts in a flexible subassembly and other parts in the main assembly are defined in the main assembly.  This is evidenced by noting that the "Mates in subassembly" folder doesn't include the mates made in the main assembly; and conversely, the "Mates in main assembly" does include those mates.

Now that I have the subassembly parts in the desired position in the main assembly, when I switch over to the subassembly file, the parts that I had positioned in the main assembly are no longer in the correct position.  How do I make the subassembly maintain the same relative positions of the parts as they appeared to have in the main assembly?

I tried re-setting the subassembly to "rigid" after positioning its parts in the main assembly, but that doesn't work.