Ah, I understand what you mean. I was in a similar situtation--we used our instance prior to the sync as well. For the leads that had email addresses, it appended the data, but the lead source was whatever program we did in Marketo, and not whatever was in Salesforce. It won't create new leads for those email addresses you've already sent to, it'll just match but won't overwrite lead source/program source.
Hope that helps!