Originally Posted by
tds101
"Freewheels and freehubs are both mechanisms on bicycles that allow riders to coast while riding. The main difference between the two is that a freewheel's coasting mechanism is built into the gear cluster, while a freehub's coasting mechanism is a sub-assembly of the wheel's hub. Freehubs are more reliable and efficient than freewheels, and allow for faster cassette swapping."
It could possibly be a freehub (after further analysis), but it's still not possible to simply remove a freehub and replace it with a cassette. They're not simply upgradable, as far as I know. Correct me if I'm wrong here...
IIRC, 5 speed freewheels back in the day were fairly swappable, similar thickness and same threading onto hub, though I could be way wrong about that, I only used a couple and never replaced.
With freehubs and cassettes, they are easily swapped, If and Only If, the two are compatible. I began with Uniglide 7 speed, high cog threaded, no lockring. Next set of wheels had freehub threads outside and inside, to accommodate UG and Hyperglide with lockring. Then the next set of wheels came with a bit longer freehub body, probably 8 speed, wouldn't fit in my 126mm dropouts, so I put the old freehub body on the new wheels. Now, my mid-00s Dahon 7 speed, the hyperglide freehub body is as short as Uniglide, but has a shoulder on the end to allow 11T small cog. If I want to upgrade to 8 speed (planned), I need to change the freehub body, 8 speed is very slightly longer. I think 8,9,10 bodies, maybe even 11, all use the same length body, just the cogs and spacers get skinnier. I know sheldon brown has a whole section on this.