Actually, the $20 migration from Win98 to Win98SE was an "update", not an "upgrade". Truly! That's MS-speak, and they decided there's a difference between those terms, between those products.
'Ran into big problem on a friend's system, and re-installs of SE (update) didn't touch it. MS said the fix required re-installing Win98 -- that an update disk wouldn't fix this! They were certainly right in this instance.
I've installed ME (upgrade) on a system, fantasizing it might solve a problem: no such luck. It's noticably slower starting programs, and has been more irritating than SE, so save your bux unless you've a demonstrated -need- for it. 🙁