Keeping a constituent's original source when calling UpdateAccountInfoEx2()

Former Member
Former Member $organization

How do I preserve a constituent's original source when calling UpdateAccountInfoEx2()?  According to the Web API Documentation V10.0 on page 425, passing zero "use[s] the default original source".  Does this mean that it resets the original source to the web default?  If so, how can I retrieve the current original source ID to pass into this method?  GetAccountInfo() returns the description of the current original source, but not its ID.

Thanks,
Bryan