ArcConvert is a free portable tool for converting archives between various formats.
The program can read these formats: ZIP/ LZH / CAB / ZIP / ARJ / ACE / RAR / TAR / TGZ / GZ / Z / BZ2 / YZ1 / YZ2 / GCA / BEL / RPM / DEB/ BH / Noa32 / HKI / PAQAR / SQX /HA /ZOO /UHARC /LFB / ZLIB / UCL / IMP / RS / SPL / APK / Arc / DZ / MSI / ALZ / PMA / PAQ7 / CHM / UDA / PAQ8 / Cryptonite / ISO / LZOP / BMA / ZIP AES (128/192/256) / Nanozip Alpha/ XZ/ FreeArc/Zpaq/GZA.
It can convert the archive to ZIP, 7-ZIP, CAB, LHA, TAR, TGZ, BZ2, YZ1, BGA, RAR, ACE, NOA32, PAQAR, UHARC, YZ2, DZ, HA, XZ, FreeArc, or ARJ/PAQ9/GZA.
Note: RAR and ACE support isn't native: you'll need WinRar and WinAce installed.
One significant problem is that you can't batch convert archives - ArcConvert processes a single file at a time.
But if you can live with that, the program works as advertised, and does have some surprising extras. If your converted archive is larger you'll be asked if you want to try again with another format, for instance; if it's smaller you'll be asked if you'd like to delete the original.
There's definitely a place for a tool like this, but it really needs the ability to process multiple files at once.