What does PRO mean to you?
Because in cross country racing there are two meanings. First is having a pro license. The steps are outlined here:
Mountain Bike Category Upgrades - USA Cycling
- 3-2: Five top-five finishes is a mandatory upgrade.
- 2-1: Five top-five finishes is a mandatory upgrade. Riders may choose to upgrade with two top-five finishes
- 1-Pro: Riders are eligible to be upgraded to Pro after two top-three or three top-five finishes in Elite/Open category 1 races at USA Cycling National MTB Calendar Events
Simply put, you can get a pro license if you always finish top three.
That said, many people with a pro license do not make a living as cyclist. They just won a few races and their sponsor ship ends at the club level. The majority of guys with pro licenses are unpaid.
To make a living as a cyclist you have to be really, really good -- better than everyone at the local races by several minutes in XC races. You must be utterly dominant to even clear $30,000/year as a professional cyclist.
My advice to anyone who wants to be a pro cyclist is to get a part-time job at a bike shop because that's where they will likely work for the rest of their lives.
Is it hard to transition to downhill? That depends on your bike handling.