> Curious question: When did AMEX change from everything being due at the end of 30 days to making payment like a regular card?

Depends on the kind of card. Some like the Blue Cash series let you carry a balance. The classics like green and gold don’t. So make sure you know which one you have.

> Is there anything else I should know before swapping my main use from Visa to AMEX?

Know your benefits! If you get better rewards on one kind of purchase (say gas or grocery) then use that card for that kind of purchase.

Also if your card has AmEx offer support, keep an eye on them. Some of them can be really good, especially if you were going to shop there anyway.

Example: I’ve seen an offer for a normal grocery chain (think Kroger/Safeway/etc) for a $20 refund after spending $100 in groceries over multiple purchases. It was so easy to do that in the 90 days or whatever it was and you basically got 20% for using your AmEx.