For the "Standard" MP3s, all you are able to do is change the key up or down 1-2 steps. For me, the additional $1 to get the Custom Track is well worth the investment - you are able to change the volume levels on every track, silence any tracks you don't want included in the mix, and keep backing vocals if desired or eliminate any leads you want.... the KV site only allows you to transpose the tracks one or 2 steps in either direction, so when I need more of a key change, I use "Slow-Downer" which allows me to transpose the tracks to just about any key I want.

Another major benefit of Custom Tracks is being able to remix and re-download as many times as you want...for example, if there's a guitar part that you only want in your mix 1/3 of the time, you can download the guitar part by itself (along with the whole song with the guitar part muted) and then create your own mix using a program like Mixcraft Pro Audio which is what I use to manipulate tracks....

Randy, thanks ... from what you are describing, my guitar player uses the Custom Track ...
As for transposing into another key, I can do that on the KB ... just have to be careful with background vocals - raise them too high they sound like the chipmunks, too low they sound like Darth Vader ...
More and more I am thinking I have to look into MixCraft ...

Tony - I hear you on the "chipmunk" background singers! The site has made many of my weddings much easier for sure. And they have some great arrangers/performers doing the tracks.

Mixcraft Pro Audio is great! Been using Mixcraft a while now and the latest version even includes Melodyne pitch tools - a lot of capability for what you pay them.