The belief (and this is based on the way things had worked in the past, and may still....but I can't confirm) is that it doesn't matter which SWUs you attach to the reservation. As long as it confirms the SWU use prior to the expiration, the "system" will pull the old ones automatically.
Thanks. I called back the day I crossed 100k and they applied the upgrades without any issue; the system (or people) pulled the ones expiring this year.