Toshiba might release a FlashROM upgrade, in which case you will be able to upgrade your OS. The OS is not stored in RAM with all the other programs, it is stored in ROM or Read Only Memory. This memory can be changed but only through a FlashROM updater on your computer which the manufacturer has to provide in the case of an upgrade. Although it is a whole new OS, so it will probably cost you a small upgrade fee.
Linux is available, but it's kind of buggy to install on PPC devices because most require a dual boot memory card.

And flashing to Linux kills compatability with all PocketPC apps, of course.
If you wanted a Linux device then check out the Zaurus device. It even comes with a built in thumbpad keyboard.