Is there any reason you are stuck upgrading this? I just know the first time you have an issue and call VMware support they are going to find out you upgraded from 3.1 all the way to present and they are going to suggest a clean install. I wouldn't even attempt this unless you are a 2000+ seat deployment with a bunch of pools. A clean swing migration would be preferable.
First check a official guide of upgrade www.vmware.com/pdf/view-46-upgrades.pdf
Second, look which version is compatible to works with VMware View + Vcenter + VMware - http://partnerweb.vmware.com/comp_guide2/sim/interop_matrix.php
I thought I best come back and do a bit of explaining !
Although I agree for a small installation, it would probabily be easier to start again, this isn't a small installation. It has several hundred users over some hardware load balanced brokers. On top of this there are over 30 different pools, hundredsof users etc and 4 installations.
To redo all that by hand would be painful Also there is a lot of compliance and security aspects to it as well. One slip could be very expensive!
I have actually opened a call with VM to specifically ask the question as I don't want a broken broker! I did check out the upgrade guide prior to posting and again on Mauros suggestion but it doesn't say "Vcenter 2,5 U6 or above, it just says "Vcenter 2.5 U6"
Thanks for all the help and if I get anything useful I shall come back and post!