This is an interesting question, assuming you've bought the new license. I am unsure why you would want to go through the hassle of downgrading just because a service pack increases the overall store limit. If you've purchased Enterprise Edition, I cannot recommend that you go through the process of trying to downgrade just because that was your only desired feature for Enterprise.
There is no Microsoft provided path that I am aware of to downgrade from Enterprise to Standard Edition. And, considering Active Directory and Microsoft Exchange are so intertwined, it may actually require you to start over. Starting over would be much more of a pain than continuing to exist with Enterprise Edition in your environment.
Plus, today may be just a storage argument -- tomorrow might bring more advances that again require Enterprise Edition.
More information on the limit increase and Service Pack 2 can be found in this article, Frequently asked questions about Exchange Server 2003 Service Pack 2.
MEMBER FEEDBACK TO THIS ASK THE EXPERT Q&A:
Could you bring up an Exchange 2003 Standard server, let it replicate, move all roles over to the Standard server and then take down the Enterprise server?
I believe this will work. You just need to move everything from the original Enterprise server to the new Standard Edition box.
—Peter terSteeg, Deployment and Migration expert
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Dig Deeper on Legacy Exchange Server versions
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