There's no free "conversion license". While you can order an 1140 with either FTD or ASA image, ordering it with FTD normally includes the Threat license for (for IPS) and optionally Malware and URL Filtering licenses. Those are all paid licenses.
There is a free Base license for FTD but it's very rare that a customer would run one without at least the Threat license. If you had a Firepower appliance with ASA image and wanted to convert it to FTD you would need to change your Smartnet SKU to be the one for FTD so as to have entitlement for the FTD software download for that device.