Explanation of the error:
During a deployment, if a field type is being changed, but that field is reference by another object, Salesforce is likely to surface an error of: "Unsupported custom field type conversion attempted".
This reference may be from a Process builder, Apex class, Validation rule or Flow.
Resolution:
When this error is encountered, the object containing the reference to the object you are changing the field type on will need to be updated first.
As an example, you have a Flow that references a text field that you want to update to a restricted picklist.
You would need to edit the object type first to a picklist and save. Then edit again to change this to a restricted picklist.
Your deployment of the new restricted picklist values will then be able to proceed.
Further examples of this Salesforce validation error can be found here:
Disclaimer: This error is returned by Salesforce directly, rather than Gearset. Even so, we offer guidance based on our combined experience with the Metadata API. Where possible, we try to help guide you to fix or avoid this error. In the case that this isn't possible, we may need to direct you to Salesforce support for further clarification.