The harm in not dropping the original index is the overhead Oracle has in maintaining both indexes when only one is needed.

The downside to leaving them is a drop in CRUD operations' performance on your table, I realise from your post that this may be "insignificant" at the moment but tables grow over time and in the future this could cause you problems that would then have to be remediated.

It will also take up more storage unnecessarily.

Your previous procedures will still be able to use the new index too.

Leaving unnecessary indexes will confuse future developers and DBA's that have to support your database costing them time and effort to investigate why the duplicate index exists.

Rather than looking for reasons not to drop the original index, I'd be looking for reasons to keep it.

Drop it, test your "other" procedures' performance and you should see no difference, if there is a problem you can investigate why and if necessary replace it.

The harm in not dropping the original index is the overhead Oracle has in maintaining both indexes when only one is needed.

The downside to leaving them is a drop in CRUD operations' performance on your table, I realise from your post that this may be "insignificant" at the moment but tables grow over time and in the future this could cause you problems that would then have to be remediated.

It will also take up more storage unnecessarily.

Your previous procedures will still be able to use the new index too.

Leaving unnecessary indexes will confuse future developers and DBA's that have to support your database costing them time and effort to investigate why the duplicate index exists.

Rather than looking for reasons not to drop the original index, I'd be looking for reasons to keep it.

Drop it, test your "other" procedures' performance and you should see little difference, if there is a problem you can investigate why and if necessary replace it.