My ER balance was due to expire after 24 months of inactivity, and did so on 12/31. I had contacted them in October regarding this and they assured me that the accounts would be combined before the miles expired. Fortunately, they were happy to restore my ER balance to my MM account number (which had activity in October) with just a quick email. So I guess my accounts are also now combined, using the old MM number going forward.