There is a good article available from Microsoft on changing this to another account, How to change the service account. As for binary domain controller vs. member server, some third-party companies provide utilities to demote binary domain controllers into member servers.
It really shouldn't matter if Exchange 5.5 is running on a binary domain controller or a member server -- unless you are trying to beat the end of life support coming shortly for NT.
Once you complete your Exchange 2003 migration, you will decommission the old Exchange 5.5 servers, and undoubtedly that would include your binary domain controller loaded as the underlying operating system.
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