It is actually a certain chance but if you want to have a higher chance of getting a different raid use my strategy
Basically it makes different raids pop up.
Step 1: You go to raid and invite people put don't play the raid.
Step 2: Go to system settings and go to time.
Step 3: Then change the time by ONE DAY.
Step 4: Cancel the raid
Step 5: You get more WATTS and get a new raid.
IF doesn't work Repeat the process. O.K. Also it might change the stars
WARNING: May cause penalty on switch watch out!
Hope this Helps (: