The merchant numbers should not need to be changed. Do you have a copy of the old working page we can compare them to? In the latest version the api credentials are stored in the WebAssist/eCart/checkout/credentials/gateways.php file. You could look there to compare them to the values you had on the old pages if you still have them.
If not, you can reset them on your gateway and update them from there instead of having to re-run the wizard.