How to change a casette?

[DebW]

is the chain whip tool necessary, or is there some other tool or combination of tools that can be used instead?

There is also Pedro cog wrench which can be used instead of a chain whip:
http://www.nashbar.com/profile.cfm?c...eid=&pagename=

But it only works on 11 or 12 tooth outer cogs. A chain whip is more versatile since it fits any size cog in any position. If you have to buy a tool, get a chain whip. Or you could make your own from a piece of metal bar and some old chain.