Perhaps you are likely to see a hard pull, but it's not certain. When I've done (modest) credit line increase requests online at Amex twice this year (I had to wait 6 months between them), they didn't do a hard pull. But then Amex is known for not always doing a hard pull for exiting customers even on credit card apps. Amex does soft pulls regularly and for existing customers it may just use that data for some credit line increases and/or some credit card applications. I don't know if any other banks do this or not.

Amex Hard Pull/Account not showing on credit report.