Is it necessary to import a new complete leads list when some modifications to a few fields are required?

No, it is not required to import a new complete list. Import a new list that only contains the leads and fields you wish to modify.

Pay attention to the name you will give to this list because only the leads included in this import will be part of the list. It would be
careful to archive this list immediately after the import because it was only required for this leads update.