Just as a data point both my MR and SPG numbers are 9 digits and dates back decades.

What has been posted is SPG numbers will be changing to meet number formats in the new system, no choice. I haven't seen anything saying MR must change, but I can see there being some exceptions.

Marriott has advised temporary system outages starting today, so that may be what's biting you.

We are supposed to be able to choose which number we like best, by logging into that account and following the directions to merge our other account number. Still to be seen.

This can be a very lucrative time for phishing. Might be best to stay away from clicking links and typing addresses.