It’s been quite often that I’m requested to change the text field from single-line to multi-line or vice-versa. To achieve this usually I would suggest the creation of a brand new multi-line text field with different names and the other interfaces/integration that referring to the single-line field need to be adjusted to refer to the new field.
Today, I made a short experiment to change the text field type of a field in CRM and found a way to backup the data of the text field and reload the data once the new multi-line field with the same name using the SDK’s Configuration Migration Tool.
Disclaimer: USE THIS METHOD AT YOUR OWN RISK. PLEASE BE AWARE THAT ANY INFORMATION YOU FIND MAY CAUSE DATA LOSS, PLEASE TAKE A BACKUP OF YOUR DATA BEFORE PROCEEDS.
Then continue with Save and Export, save the schema and the exported zipped file of the record.
Notes: I tested this method on other data types and apparently the schema validation do not work.
I hope this helps!