Do you still have the Bill of Sale from Nexus when you bought it? The model year should be on it, that's what they should be going by. You should have a copy of everything that you signed. If your signed copies are different than Nexus' signed copies, then obviously they made changes without you knowing. They shouldn't be re-writing anything that you signed. If that is the case, then it might not be a bad idea to get a lawyer involved. It is unlawful to change a contract after it has been signed.
It's one thing if the DMV registered it as a 2014 instead of a 2015 - that's not uncommon if the chassis is a 2014, but if Nexus sold you a 2015 and then changed the documentation after you purchased it, that's something different entirely.
-Michael
