[linkregister]

Disclosure: I am a former employee of a Sendgrid competitor.

The proper way to transition IPs is to split traffic between them, so when one IP is throttled, traffic can continue. These accounts should have been assigned the new IP in addition to the old one. Once warming completed after a couple of days, the old removed. I would strongly recommend all affected customers to open support tickets with Sendgrid to get put on good IPs.