From the Lead's profile page, you can convert a Lead into a New Customer, or merge it into an existing Customer Profile.
If a lead is related to an existing customer (ie. like a spouse), or a duplicate record of an existing customer, you can convert and merge the lead as a new contact or address under the existing customer.
If the lead already has estimates tied to it, those estimates will not automatically be converted in the process. Instead, the estimates will also get merged and associated with the exsiting customer profile.
Leads also automatically convert into Customers, when estimates they are tied to are Won.
Let us know if you have any questions about converting leads!