So, how do I fix the imported MIDI file from automatically changing to a piano patch? I change it to Overdriven Guitar, but I'm still getting piano when I Generate the track.

It sounds like there is a program change function in the original MIDI file. A MIDI editor will list all of the data on a MIDI file and once it is listed you can delete the program change. Because it is an imported MIDI file that program change could be on any track so you may have to look at all tracks to find it.

Note I don't use RB so I don't know if it contains a MIDI edit, others will have to chime in on that.

i'm assuming you have all the midi tracks on separate tracks in RB? highlight the guitar track and press F2 - you will open an event list and you should see the program change so you can delete it.

i also had a problem once because the midi channel wasn't set and my synth defaulted to piano. if you have to set a channel you might need to experiment to find the best one

The big E icon in top row.
Event List
You want to find any program changes as Mario and Bob mentioned.

You can either remove them, or change the number to the patch you want it to be.

They are usually at the beginning of the song, so rewind before opening the Event List.
As Mario said, they may all be on different tracks (example - for each MIDI channel), or they may all be hidden on one track.

Another option is to blindly Cut all program changes.
To accomplish:
Highlight all tracks from beginning to end.
Click Cut icon (or CTRL + V)
Enable Data Filter (!important)
Hit Enter
In the data filter deselect everything but Program Changes.

Cut 'em (Enter).

Remember there is an Undo button if you goof anything up.