As all the stamps you are using now will have the barcode on them, they will be treated as though they had been intended to be used for the service they cover. Almost everything is automated; it makes no difference what you write; nobody will see that, and the automated systems will only scan the barcode. If your package is too heavy for the first class service, you will be charged for underpayment.
To a certain extent, it's "swings and roundabouts" for you - every package which is below the limit will be treated as a first class delivery, so you're getting a better service than you expect with these ones (although you have paid for it)