Windows 2000 -> 2003 upgrade kills DNS replication of secondary DNS zones!?

I just want to tell you about an odd little problem that I found during a Windows 2000 to Windows 2003 domain upgrade project with one of my customers (Actually my all-time favourite customer ;-) The project included consolidation of their Domain Controllers/DNS Servers in to fewer servers and upgrading a Top Level Domain and later their primary domain. During our planning/testing phase, we decided to in-place upgrade the excess servers to Windows Server 2003 before demoting them and reinstalling the servers on new iron ('Cause in our experience 2003 handles the process of demoting a DC much better than Windows 2000).When we upgraded the Domain Controllers in our lab environment, we found that the secondary DNS zones on these servers didn’t receive any updates from their primaries. Oddly there weren’t any errors during normal operation or when using “Transfer from Master”; but zone transfers first started working when we deleted/recreated the secondary zones. At first we thought this was a “lab error” – but during the real world implementation we checked this again and found the same problem (Although this time we found that “Reload from Master” and a little patience also solved the problem).