Why don't you just make the switch at BMW, I mean go into the M lease and return the Z4 35is? Or is that not possible in the US? I guess you would loose all you put in plus the $2000- and lease trnsfer cost.
When I switched back to the M from the Z4 35i, I just traded the Z in and paid the difference on the new car. This way I got the tax break (paid tax only on the cash difference). I also avoided all the tire kicking.
Anyway, good luck with the transfer.
PS. I know what it feels like, when I was younger and our daughter was born I gave up our Firebird for a four door ford