If you change the data type of any custom field used for lead conversion, that lead field mapping will be deleted:
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A. B.A.
When you convert a lead in Salesforce, the data from certain fields in the lead is mapped to fields in a new contact, account, and/or opportunity record. These mappings are defined in the Lead Field Mapping section of the Lead Settings page.
If you change the data type of a custom field that is included in the lead field mapping, Salesforce will delete that field mapping. This is because changing the data type of a field can result in a loss of data, and Salesforce wants to prevent data loss as much as possible.
For example, let's say you have a custom field called "Industry" on your lead object, and you have mapped this field to the "Industry" field on the account object. If you change the data type of the "Industry" field from a picklist to a text field, Salesforce will delete the mapping between the lead "Industry" field and the account "Industry" field.
To avoid this issue, it's recommended to review your lead field mappings before making any changes to custom fields used in the mappings. If you do need to change the data type of a custom field used in a lead field mapping, you will need to recreate the mapping after making the change.