I've never seen an extra charge for additional people while booking my stays (in the US). I've heard here a ($10?) difference in rates between a King and Double bed room at some SHS's. I don't think anything is set in stone Marriott wide and any increases would be property specific. I think you are OK esp with a revised confirmation record. But then again, I never bother to change the # of people whether its me alone or the entire family and I've never been challenged.