Christian - your existing LPs - how are they getting their info into SFDC?
You might be able to do an API swap in the background so you don't have to completely rebuild.
Ultimately you do want the activities captured in Marketo - ideally before they go into SFDC so you can nurture, score and do some hygiene, etc.
Key element is to ensure that you get the Munkin cookie associated with the email address.
You will probably want to put together a transition plan - this is what you do with all new LPs going forward, this is how you are handling priortized legacy, this is the EOL path for underperforming LPs, etc. Depending on your volumes you might take a different path. You could back-in from SFDC but not sure if it is worth the effort - not to mention it will clutter SFDC.