I would stick with having zencaster.com redirect to zencastr.com, and keep your current name/trademark. I think this is a fair compromise and probably your best bet.
For what it's worth, I prefer Zencastr over Zencaster.
I strongly agree with you. After Paul Graham's 'Change your name' essay lots of startups have been having sleepless thoughts about naming. Dude just offer an amazing service! When you hit product market fit, then maybe u can change your name! then, ur users will follow u. Product market fit is the key. I recently decided to stop being bothered about my http://classroom.ng (unified and uncluttered inbox for educative email subs you care about and can't miss), because no one will see a .com fro less than $10k ... Focus on users. look at Yahoo! Who knew?
For what it's worth, I prefer Zencastr over Zencaster.