I know how to write my own riffs but I don't know how to switch into another riff without making it sound bad and both riffs are on the same scale. Can anyone help me out?
Try to come up with a 3rd riff that bridges it well. Do the 2 riffs you have change pitch greatly? You could use a slide of some sort, or maybe just a slight pause w/drum or bass fill.
Fanatical follower Zeppelinism,

Exerpt: "... and on the 7th day Jimmy Page said let there be ROCK!!!!!!!!!"
You could try to just make another version of the first riff. They will sound the same yet totally different . The transition between such riffs should be easy.
My Setup:

Epiphone SG G-400
Epiphone Explorer
Ibanez RX
Gardisten Aucustic
Line-6 Spider III head
Line-6 4x12 Cab
Zoom G1 Multi-effect(sucks big time!)