Buy a similar disposer (eg. insinkerator 500 or badger 5), then just swap it. No need to replace the flange installed in the sink. Since you have a dishwasher connection, make sure to knock out the tab in the GD dishwasher port (hammer and screwdriver), then turn it upside down and shake until it falls out.

Never heard of a "GD installation kit" and I've installed plenty. I'll have to look that up just to see.

Here is the garbage disposer installation kit

http://www.homedepot.com/p/DBHL-Plastic-Garbage-Disposal-Installation-Kit-HP9104/202882849

You won't need this kit to replace an existing GD. You should probably take it back.

Ok. I just thought it would be a good idea to replace the existing plastic pipes with ones that come in the kit after 12 years.