When changing a transmission in supercharged 1995 Buick Riviera do you have to take out the engine?

No, you generally do not have to remove the engine to replace the transmission in a 1995 Buick Riviera, even if it's supercharged. While it's a complex job requiring significant mechanical skill, the transmission can typically be removed from underneath the vehicle. However, it will be a much more difficult job than in a vehicle where the engine and transmission are not so tightly integrated. You'll likely need to disconnect numerous components, including driveshafts, exhaust components, and possibly some engine mounts to gain enough access.