Leads vs. Contacts - where would you start?

Leads vs. Contacts - where would you start?

New to Zoho. New start-up company. New everything. We pulled data (accounts & contact records) from multiple sources to begin prospecting. Should we import the data as Contacts or as Leads? If the answer is Leads, I have already imported as Contacts, therefore, is there a simple path to change Contacts back to Leads and begin the lifecycle from there? If there is a simple way to change Contacts back to Leads, can all data associated (emails, calls, tasks, etc.) be brought into the Lead record?  Thanks.