I am upgrading an undersized bathroom fan that is 50 CFM and has (possibly) a 3" to 4" exhaust pipe. The new fan, which is 150 CFM requires a 6" exhaust pipe. Do I need to rip out the existing vent pipe and replace everything with 6" (including the roof vent) or can I get by putting a vent reducer (from 6" to 4")?
You can install a reducer, but you'll cut your flow volume by an equal proportion:
28.3in2 - 7.1in2 = 21.2in2 (an area reduction of 75% when going from 6" to 3" duct)
This will negate a significant amount of your fan upgrade, will make it work harder, and may shorten the motor's life due to reduced cooling.
You might ask yourself whether you really need 150 cfm (and the accompanying noise).
Let's back up here for just a second and note something
You almost certainly don't need a 150CFM fan.
So your original fan was a stock 50CFM (probably NuTone or one of its predecessors) that's a 4-sone fan, meaning it sounds like it's been cleared for takeoff when you flip that switch. So you went out and bought this bad boy and though "I'll solve this right now!". I bet it cost you a lot as well (that's one hefty motor there).
Anything 100CFM or more will have a 6 inch vent, but you you need to look at the size of the room. About 1 CFM per square foot should do the trick. Since it's rare to have a bathroom over 100sq. ft., you're building overkill. Downsize to a 70-90 CFM fan. You'll probably save money over the old fan and it will have a 4" vent.
Make sure your exhaust line is insulated as well! A lot of older fans had bare metal ducting.