Don't store the Axim away, jokes. Sell it if you're not going to use it, its value will decrease over time. However, if you really want to upgrade the ROM via SD have a look at
this thread over at AximSite.
You may also want to look at
this page from XDA Developers if you want to go to WM6.
I have removed the other two duplicate threads created on this subject since they hadn't contributed much to the solution (but good job dahawthorne for trying in one of them).
Mahstah