I actually found out why this doesn't work--not that it hasn't been answered already. While I was digging around (original ROM), I found that the original Sprint Navigation is in a folder called TeleNav, whereas the new installation goes into Sprint Nav. The old software never knew that the new software was installed. Here's what I did...
NOTE: NOT RESPONSIBLE FOR DAMAGE DONE TO YOUR PHONE BY THIS METHOD!
I first downloaded the
Sprint Navigation update to v2.1.23. The new version (now) is v2.1.26.
I deleted the TeleNav folder from \program files\. It will argue that it's a program--deleted it anyway.
Once uninstalled, I navigated to \Windows\Start Menu\ and deleted the Sprint Navigation shortcut.
I then installed Sprint Navigation using the .cab file downloaded previously.
Once I re-registered and launched v2.1.23, it advised that there was an update (may require shutting down/restarting once to get message), but no option to "download later".
Downloaded the newest update and installed it. It uninstalled (unsuccessfully in my case) the older version, then installed the new version.
Once the new version was installed, I re-registered once again, and checked the version I had under "About"--v2.1.26 installed!
Now, this is my experience on my Treo Pro. Yours may be different. The programs are identical as far as I can tell.
Hope it works for you like it worked for me!