Short version: You can convert high def to standard def, but not vis-versa. ("It has always been easier to destroy than to create.")
Back in the 60s, all the VFX were created in high def, on film, because they didn't have any other way. Starting in the 80s, with the rise of tape and CGI, that changed; effects were created in standard def to start with, and they never had a high-def version. After all, the shows were only going to be shown in standard def, right?
The Twilight Zone (and the original Star Trek) were made with the older methods, so a high-def version already exists. But for Babylon 5, that standard-def version is all that was ever created. There's no way to get a high-def version without starting from scratch.
(Star Trek: The Next Generation fell in-between the two methods. The final version only existed at standard def, but they still had most of the original pieces in high definition; CBS has been able to reassemble them into a high definition version. For B5, those original high-def pieces don't exist.)
