First question........ you DID register the activation code for the Lifetime maps on the unit, correct? Either online (doesn't work well) or by calling CS at 866-486-6866 (preferred method).
Once the activation is done, TomTom's servers associate the serial number of the unit with the email account of the user regarding the lifetime maps.
And here is where the problem arises. The Lifetime maps are not meant to be transferable. So the new user could continue to log into TomTom if provided with the email address/password used by the original owner of the 540.
As I see it, the only other way is for the new owner to get an understanding customer service rep who will transfer the map information to an account set up by the new owner, but I don't think that's an automatic assumption.
Best a call to CS is made. To clarify the situation.