With SugarCRM you cannot update existing contact/lead records via the import wizard using email address as a sync point. The import wizard will only update records using ID as a sync point.
This is a very common need for us. The standard use case is a bunch of our contacts attend an event, and we then want to mass update those records via a list import--to add a tag indicating they attended that event so we can create target lists for follow ups, for example.
I dug into the documentation to try and find a solution and I was a tad mortified to discover Sugar's suggested workaround: using VLOOKUP in Excel to populate the record ID into the spreadsheet prior to upload.
This seems insane to me. Rather than building this capability into the platform, they are suggesting a user keep an entire copy of their Sugar data set on their local machine. That's a MASSIVE security risk. Plus, if you have a large data set, the average business PC is going to struggle mightily with processing this type of function.
Seems like it would be relatively easy to add this functionality to the import wizard and would be very helpful for my company.