On the last booking I made for Mrs DMF and I, a MFU booking, I was a Silver cardholder and this allowed us to select seats on all flights even though we were returning on different flights/dates.
Before we took the flights I dropped back to Blue, sob sob
, but our seat assignments remained in the booking. I did not try and change them as I was sure this would lead to loss of the seating assignments.
So if you are Silver now you could assign seats for the return J flight and if your experience is the same as mine they would remain even after you have dropped to Blue, this will of course not work if you change the date of your return.
Hoping this will be helpful
DMF