Exchange Upgrade from NT4.0 sp5 to sp 6a

We are about to upgrade to sp6a on all our servers in our Exchange domain(nt.wwh.witco). We run in a clustered environment with 3 additional servers. Right now all servers are running Windows NT 4.0 sp 5 and Exchange 5.5 sp 3. I would like to know if anybody had any issues when they upgraded to sp6a. We do not have a test environment to test this out it.

Nancy Anderson