If you are an administrator you can try to use Scriprunner plugin (ti is free) to perfom such actions.
You can use out-of-the-box functions (e.g. Copy custom field values) or prepare them on your own.
Description of the Copy custom field values function:
Copy custom field values from one field to another in bulk (to support changing a field type without using SQL or restarting).
How to find where it is this action:
1. Go to Administration pages
2. Select Add-ons tab
3. On the left side in Scriptrunner section select Builtin scripts
4. Click on Copy Custom fields values
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