DON’T DO IT!!! (well, not unless you’re just trying to see this for yourself).
Changing the “log on as” credentials for a BizTalk Host Instance service in the services MMC doesn’t change the account that is registered against the Host Instance in the BizTalk databases. If you take a look in the BizTalk Administration Console you’ll see that the credentials are still set to the old ones. I did this on a Windows 7 Enterprise machine which was running BizTalk Server 2010 Developer edition.
I don’t know quite what the impact of doing this would be, but I’m not brave enough to find out 🙂 If anyone else is up for the experiment please let me know your results.