MarcM, I am in a similar situation. We also bought the additional acrylic cover in order to retain heat for difficult-to-print materials, fully expecting to be able to use it (and the new improved aluminum build plate) to print 'challnging-to -print' materials like PC. Without the improved build plate, we would have been MUCH better off purchasing a less expensive (but no less capable) printer.
I think it is the obstinate position taken by Ultimaker that this decision to cancel the aluminum build plate is no big deal. That decision is truly in the eye and mind of the ($6k USD poorer) purchaser who may not be able to do with the machine what they were led to believe they would be able to do with it. I have not yet engaged Ultimaker, so I cant yet say how my specific case will be handled, but I will be sure to report back once I know.
I have been thinking about how best to proceed with Ultimaker, and will be in contact with them shortly. I hope they are willing to do what is necessary to make this right.