ffmpeg - How do I convert audio while preserving its bitrate?

Question

I want to convert audio downloaded from YouTube (in OPUS codec) to OGG, but when I do this using the basic ffmpeg -i -vn <file> command, there is a slight decrease in bitrate (as an example, 131kb/s to 115 kb/s). Is there a way to prevent this without manually checking the bitrate on each file and enforcing it in the command?