Just posted this info on Beermountain, so I thought I would share it with the good folk here.
Having just changed my car, I needed to amend my ferry booking with Brittany, no problem, just get on the website and click on the manage my booking tab, OH NO!
The website tried to charge me an extra £180 for taking a similar sized vehicle. I phoned Brittany Ferries, and the lovely Louise explained that changes made on the website will reset the entire booking to reflect current prices, not the price when originally booked, hence the huge hike in cost. The amenment was made over the phone, and no costs applied.
Don't rely on the web, make that call!
Slash