Open Source JPEG2000 libraries:
- Grok, last commit 9 hours ago: https://github.com/GrokImageCompression/grok
- JasPer, last commit 5 days ago: https://github.com/jasper-software/jasper
- OpenJPEG, last commit 8 days ago: https://github.com/uclouvain/openjpeg
I didn't try them, but I think you can probably still read your 20 year old JPEG2000 files today without too much problems.