There is no need to upgrade for exisiting Sinarback users wishing to use the Sinar Hy6 camera: all one has to do is to change the adapter plate, with the exception of the SB 54H for which an evaluation is in process.
However, there is currently a trade-in promotion running since June '07 and until October 31st (lickely prolounged): for those who wish to buy a "Sinar Hy6 - e75" system, Sinar will take back their current MF camera (incl. one lens and viewfinder). For details consult your local distributor.
I'm sure most Sinar users would be interested to know this too.