If you’re already using Cloudflare for DNS, you can drop TTL for those DNS entries down to 2 minutes. Right now, it’s probably set to Automatic, which is 5 minutes. Whatever it is (it could be as high as 24 hours), that’s how long you will have to wait for your new TTL to take effect.
At worst, some email will still trickle into those Office 365 accounts, so keep them open for a week and use their webmail interface to check for stragglers.