Changes to classes can be very disruptive for many people, as they may negatively affect their main or even multiple, if not all, characters. A lot of effort may have been put into optimizing their gear for older builds, making any changes heartbreaking for some players we all know and love.
Additionally, some older classes and races have been grandfathered, even though they are no longer playable.
So, instead of forcing older characters to relevel, why not just let them be and provide it as an option instead?
P.S. I understand that some mechanics are being reworked at the code level, but would it not be cool to save and have some "old-school" characters running around as well?