I booked the ferry last month, but since then I have changed my car, does the ferry company need to amend the details, or will the check-in at Dover issue a boarding pass for the new registration?
LKast time I took the ferry was 4 years ago and once or twice we decided to go in the wife's car instead of mine which the crossing was booked for, but it never was a problem then.
I would just call them to make sure. They shouldn't charge you anymore for it.
We have let them know in advance before and they charged as an admin. fee, if you just turn up at the terminal and let them know the other car wouldn't start there is no fee.
next time you book, put "not known" in the reg number field. Never been a problem for me at check in and leaves car choice open to you till the last moment.
I changed my car over the phone with norfolk line a few months before we left and they didnt charge anything. I would phone them and change it for piece of mind