41 levels difference between transcendent (what we have now) and the 3rd classes on a regular rated server.
The max level went up to 150 instead of the old 99, that's a difference already that would be hard to convert.
There are new skills that are based off joblevel, on a HR such as this one, that can be devastating for gameplay.
Testing this with a few people will be fairly useless. Okay, it's alright for PvP action, but consider the same for WoE, GvG and other guild/team related events, you'll figure out that it's hard to balance, heck, we're still trying to make the current classes work the way that would provide the most balance in game play.
However, the idea of a test server itself to test possible new features can be a good thing, but to aim at 3rd classes straight away and go with renewal isn't an option as far as I can see.
Plus, yes the Development team has their own test server(s), but it's running on our own computers and will only be active when we test ourselves.
So far, I think Katherine tackled the issue straight away and also clarified why the server is the way it is and most likely (for the distant future of roughly a year) will be like we know it now.