Sync Lead to SFDC Call

I am setting up a program that will adjust lead scores in the database. If I set up the flow to be:

1. -20 Points

2. Add interesting moment for score calibration

3. Sync Lead to SFDC

Will leads currently in the Marketo database that are not yet in SFDC be created in SFDC? Or will only existing leads sync to SFDC and the remaining leads update in Marketo.

Thanks for the help.

ML