You should create new field with List data type and then run fix script to copy data from choice list to list field.

Run your fix script in background so that performance is not hit and you can monitor using progress worker.

This way you don't loose any date in existing choice list type in PROD instance.

You will have make sure that all your objects ( like business rules, client scripts , script includes) are updated to accept this new data type change so that your functionality is not implemented.

Also, make sure that you do this first in DEV instance and then move to production.

Regards,

Sachin