AAC is one of the most widely used and implemented codecs in existence. It’s extremely unlikely that you’ll ever need to transcode it for compatibility since the only reason to do so (licensing) becomes moot over a preservation timeframe as patents expire. If you aren’t noticing technical defects in your recordings now, you’re really not going to care in 20 years - many of us will be lucky to still hear well enough!

> And for archival of digital source material, lossless is the only sensible option.

I work in digital preservation and one thing everyone is keenly aware of is the cost of storing large amounts of data. Archivists like lossless, of course, but if it’s below the level where humans can notice problems they’re often more forgiving than you might think because preserving twice as many works with the same storage budget has its own appeal. Your personal music collection is probably small enough that it fits in your personal storage slop but maybe that’s not true if you’re a huge fan or partial to high-res soundboard recordings, and if you’re archiving for a larger group you’ll start to exhaust what capacity you can get for free.

> that’s not true if you’re a huge fan or partial to high-res soundboard recordings, and if you’re archiving for a larger group you’ll start to exhaust what capacity you can get for free.

But hard drives are cheap. I have a multi-TB hard drive that is used exclusively to store my music. When that fills, I'll just get another.