How early can I report my 90 days in Thailand and renew my re-entry permit?

Q. How early can I go to CW for a 90 days report?

-> reporting window for 90-day report in person at CW is from 15 days before (same day of the week in two weeks earlier) until 7 days later.

CW provides an online appointment for a 90-day report, too.

Q. Can I do that as well there beforehand or does it have to be, once it’s expired?

-> The expiry date of the Re-entry permit is always the same as your current extension/entry stamp, regardless of when you apply for it. Unless you get a new extension in advance, your new re-entry permit is still valid until mid-April only. If you already have a new extension longer than mid-April, you can apply for a new re-entry permit matching your latest extension anytime, even before mid-April.