How to Change UDF Type in Requisition Fields?

Summary

Is there a way to change the UDF Type in Field Editor for a current UDF in use?

Content

Hi experts, 

Some of our UDFs in Fluid have transferred across poorly from Taleo so we are looking to change the UDF type from 'Number' to 'Text' to see if this would help the formatting issues in the UDFs.

However, when in Recruiting Fields in Recruiting Administration, and when editing the properties of the UDF itself - the type field is greyed out (see attached screenshot). Is there a way to be able to change this type? 

 

Thanks!