Hi
I don't think there is a customized upgrade.zip from any Vendor.
Basically the custom ESXi ISO is the Standard VMware ESXi5 ISO with Vendor specific drivers install as "bundles and then repackaged in a Custom ESXi5 ISO
You would need to use the normal upgrade route to get to ESXi 5 and then just apply "esxupdate" the custom bundles "drivers" that are available from the vendors sites.
My 2 Cents though
I like to start fresh whenever possible, then you know you are not inheriting old problems. For example upgrading from Win98 to WinXP to WinVista and then Win7 this is sure to end up as a mess and a lot of time and effort.
The vCenter I would upgrade the DB to keep the data, but install vCenter 5 on a new fresh Win 2008 OS and point to the upgraded DB.
Just make sure these hosts is on VMware HCL before you upgrade/reinstall :smileycool:
Please don't forget to award points on your previous discussion http://communities.vmware.com/message/1851857#1851857 
Also please award points if you find this helpful/correct 
Regards
Pieter