I am afraid that I will lose my data if I convert a Lead to a Contact or a Deal. What should I do?

The best way to save your data during conversion is through field mapping.
Vtiger CRM helps you map custom fields in the Leads module to Contacts, Organizations, and Deals modules. So when you convert a lead to contact, organization, or a deal,

• the CRM considers the field mapping,  
• transfers field values in the Lead record to the selected target fields in the Contact, Organization, or Deal record. 

To learn more about Lead Conversion Data Mapping, click here.