How can one best prepare for that going from CFI to 121, without any 135 in between?
There's three very different skill sets you have to master when you go from a CFI to an RJ Monkey.
The
first is related to the actual airplane. Pretty much no matter what you are CFIing in, whether it be a single or a twin, glass cockpit or steam gauge, the RJ is going to move faster, have more complex systems and operate in airspace/procedures, you've never experienced before. Additionally, the way you are going to be taught all of this stuff is via a fire hose being stuck in your face, and them opening the nozzle. The best way to prepare for this is to
a) Make sure your self study skills are solid (which has absolutely nothing specifically to do with flying).
b) Make sure your outside of work life is in order so you can focus 120% on learning the new plane.
c) Start learning about, generalized, more complex aircraft system, often found in RJs (APU, pneumatic, hydraulic etc). There are several good books for this out there including, the Turbine Pilot's Flight Manual. The first time you hear the words "low pressure turbine", shouldn't be in ground school.
d) Make sure you are super instrument proficient because the procedural flying stuff has to come together automatically in the background while you struggle with the new airplane part at speeds WAY faster than you are used to.
The
second challenge of going from CFIing to working in an RJ is understanding the multi crew environment and getting a handle on PM/PF and CRM interaction. You can start to wrap your head around this by reading some books and talking to people who fly in that environment every day. You can work it into your lessons with your current students, especially ones that are planning to go fly multi crew airplanes eventually.
The
third new thing you'll encounter in your first go at a 121 op will be airline bureaucracy and the labor/management relationship. If you've ever worked for a big company before, you'll have seen most of this, but if not, grasping that you are just your employee number and not much else (to start with) can be tough for people.
I lied... there are actually four things, but one of them won't come into play for a while. When you upgrade, knowing how to be PIC and actually be in command of an aircraft with people in the back (and everything that comes with that), is a skill that can not be taught during 3 weeks up upgrade ground school. The time to build the PIC skills is now, while you are instructing and then once you get comfortable in the right seat of an RJ, start trying to thing like a captain. That doesn't mean BE a right seat captain, but stay involved, try to come up with solutions to problems before the captain does and see if what you thought of, is what they think of. Always be learning and expanding your knowledge.
Going straight from being a CFI to a 121 pilot is very achievable, especially as they've catered more and more RJ training programs to that reality. You've just got to put all your effort in to it.