Leads/Deleting Contacts

We have just recently began using leads here at my work. In the past, instead of creating a lead, the user would just create an account and name is Company A Reserves. This was a designation account for all leads at that company. Now that we will be using leads, I've exported all contacts from these reserve accounts and imported them back as leads. I now need to delete the contacts, but I do not want to lose any activity history. How can I do this? I tried to simply merge the Lead with the Contact, but CRM recognizes the