Quote Originally Posted by OakLeaf View Post
You might give us longer than overnight ...

I think the hesitation is that it is pretty simple, but we're not really familiar with your degree of mechanical expertise or what other components are on your bike.

There are two special tools required for removing a cassette - a lockring remover, which is like a splined cylinder with a hex head, and a chain whip, which lets you hold the cogs so they don't spin while you're loosening the cassette. They're good things to have and know how to use anyway, since cassettes are a wear item that will need periodic replacement, plus you may well want to swap back and forth between two cassettes if you ride in different terrain.

I think any rear derailleur that will accommodate a 25T low cog will also accommodate a 27T and possibly a 28 (someone correct me if I'm wrong). If you want to go bigger than that, you probably will need to replace your rear derailleur with a longer-cage mountain bike derailleur. At a minimum, with any cassette change, you'll need to re-set the B-screw on your rear derailleur, and if you make more than a 3-4 tooth change in your big cog, you're likely to need a longer chain, too.

The short answer is there is no short answer with the information you've given us. How comfortable are you with tools? How much would you like to learn? How much do you trust your local bike shop?

There's a lot of really handy information on parktool.com and sheldonbrown.com .
I think my original statement in my post was obvious: But I don't know the first thing about cassettes and all of those gear ratios (mine is a 11-25T).
So I was looking for a basic explanation of what all those numbers meant. Also I was asking if they were easy to change out therefore getting an idea if it was something I could do or do I need my LBS to do it. Thanks.