I’d be surprised if there wasn’t an FFMPEG command to run in PowerShell to automatically convert all of them.

In Linux I always use sound converter. Works great.

I wonder how much storage I’m going to save converting my mp3 library to opus

Depend on the source and output quality you have. Also do not forget conversion from lossy into lossy format will degrade quality too, even if its most of the time not noticeable. If you have them all in constant 320 kbit/s, then you could probably get a good chunk of space without sacrificing quality much.

As for the software recommendation, it would be good to know what operating system you are on. Windows, Android, iOS, Linux PC?

Program: FFMPEG

Format: .flac w/ FLAC audio (or .mka w/ Opus audio if you really need a lossy codec)

Also, the amount of storage you’re going to save (or lose) by converting from one codec to another (ex. MP3 to Opus, like in the post) depends on the input and output files’ bitrates. But, the quality of the output file will always be worse than the input file if the output file uses a lossy codec.

SoundConverter works great for batch conversions. It will convert multiple files at once to speed things up.

Opus is considered lossless transparent at 128k VBR, so it will use less than half the space of the 320k MP3s.

Don’t convert one lossy codec to another, it will just make it sound worse. You need to re rip them from the CDs or download them in FLAC.

I use fre:ac. It is open source but I’m not savvy enough to see what the license is.

I don’t use MP3, I use AAC. Never liked Opus. I get it’s a free/open codec, but I’ve never liked the format. So I’m not sure what the licensing is for AAC. But I’m sure fre:ac can do Opus as well.