Update Salesforce accounts only

I'm not entirely sure what you are trying to achieve. I assume you mean that you converted the lead in to a contact (attached to an account). We have a similar process. On our end we made sure Marketo can't update account fields, only contact fields. There is just to much critical stuff on the SFDC end.

If you have a similar set up, you could consider updating fields on the contact level. You can always make workflows in SFDC to update the account fields based on the contact fields.

Thanks so much for the response Diederik! Sorry it wasn't clear but what you explained above seems to be pretty close what I am hoping to get from Marketo. 

Our issue is that there might be some instances where a lead is created and then instantly converted into a contact. We would love to still have that contact interact with (for example) a Marketo form on a webpage and have the data from the form go directly to the contact profile. My fear is that marketo will create a new lead profile since the lead was potentially converted. Is marketo smart enough to know that a contact exists in salesforce and that it shouldn't create a new lead? Maybe I am thinking about how marketo creates leads in a totally wrong way.

Thanks!

If a lead is converted to a contact in SFDC, it still has the same Marketo ID. Considering Marketo has at least read access to SFDC contacts. Just test it with SFDC Type [Person] = Contact and see if you have any of these records in Marketo. Marketo will never create a duplicate email address.