This is one of those situations where almost everything you try will be overcomplicated. In Preferences-General there is an option named: Force lowercase filename extensions when renaming. Make sure the option is enabled.

Define a rename template which is defined as in the attached screenshot. The duplicate handling is really only required on case-sensitive file systems but it is safe everywhere. Personally I always supply duplicate handling.

Simply rename a file using the Current Filename rename template. If the filename extension is already lower case nothing will be renamed as there is nothing to change. If the filename extension is not all lower case it will trigger the rename. The filename will not change but the extension will be written as lower case.

The Force... option works with any rename template.