Hi John,
Here is the answer to your question:
Yes. You can decommission both the Exchange 2003 and Exchange 2010 servers.
More information:
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After you’ve verified that all e-mail is being routed directly to the cloud-based mailboxes, completed the migration, and no longer need to maintain your on-premises e-mail organization, you can uninstall Exchange from your servers and remove your on-premises Exchange organization. For detailed steps, you can refer to the following articles:
How to Uninstall Exchange Server 2003
http://technet.microsoft.com/en-us/library/bb125110(EXCHG.65).aspx
Modify or Remove Exchange 2010
http://technet.microsoft.com/en-us/library/ee332361(v=exchg.141).aspx
Thanks,
Monica Tong