#1
Ive been trying to keep my feet wet in the area of modes lately despite the fact that I think most blues and rock guitarist only use the 1st and 6th degree (correct me if Im wrong). Anyway, a couple examples keep popping up in songs Ive learned and I dont know the theory behind it and my best guess is modes. Example:

Heartbreaker - Led Zeppelin (Main Riff)
Running With the Devil - Van Halen (Solo)

There are other examples that escape me at the moment, but in those songs they play a certain riff and then simply slide it up or down two frets and play the exact same pattern. Is this mode changing at work or something else entirely that I havent though of? Thanks for any help.
#2
It's modulation. You take a riff and move it into a different key. It doesn't have to do with modes.

This is very common. The "Day Tripper" riff starts out in E, but then moves to A for a few measures.
#3
The most common thing in this is the blues riff. It will start in E:


A-2-2-4-4-5-5-4-4-
E-0-0-0-0-0-0-0-0-


Then Move to A:


D-2-2-4-4-5-5-4-4-
A-0-0-0-0-0-0-0-0-


Then you'll play it in B:


A-4-4-6-6-7-7-6-6-
E-2-2-2-2-2-2-2-2-


Does this help or do you want additional info?