Rename custom field

Someone I know is kindly hosting my WordPress site, which means I don’t have direct access to the database. This isn’t a problem in most cases, but when I searched for a way to rename my custom field, the only solutions I find seem to direct me to go to the database directly.

There must be a plugin somewhere that will let me do something so simple. I have two custom fields with slightly different names. I need them to have the same name and there are far too many entries with these fields for me to do it post-by-post.

(Just to make sure we’re on the same page: I need to rename the fields, not the values.)