Mountain Biking - 7-9 speed conversion

hello, i'm new to this forum and was wondering if someone could help clarify a few things.
I have an old 7speed diamondback mtb from which the chain recently broke off. I was considering converting to a 9 speed and have looked through all of sheldon's advice but i'm still confused about whether i have a freewheel or freehub. The cassette says shimano hyperglide c on it if that helps to identify which it is.
the frame spacing on my bike is 135mm which doesn't seem to make sense for a 7speed. If i buy a 9 speed hub with a 9 speed cassette and 9 sp shifters will i need to do any other mods?
Is it neccessary to change the hub? if so could someone explain why as this isn't really explained anywhere.
Most of the mods seem straight forward except replacing the hub which seems to involve a wheel rebuild... is this easy to do?

many thanks

the frame spacing on my bike is 135mm which doesn't seem to make sense for a 7speed. If i buy a 9 speed hub with a 9 speed cassette and 9 sp shifters will i need to do any other mods?
Is it neccessary to change the hub?

135mm rear spacing is the standard that was rolled out in 1989 along with 7 speed hyperglide. The whole "7 speed MTB is always 130mm" is an ugly internet myth that just won't die. There were a few frame builders that stuck their heads in the sand and hoped standards would never change, and Shimano accomodated them by offering the new seven speed groups with 130mm rear hubs for a limited time, but they all had to face the music eventually.

You could change the freehub body on your seven speed hub to upgrade to nine, or buy a new rear wheel. It would probably not be cost effective to lace a new hub up to your old rim.

You could also try Sheldon's "eight of nine cogs on a seven speed freehub body" and keep your rear wheel intact. Don't forget you'll need a nine speed chain too.

To answer your question as to why you need a different hub- your 7sp freehub body is too narrow to fit a 8 or 9sp cassette.