Yes, if you are initially assigned to the E120, you can be bypassed for the CRJ upgrade, but the company must pay protect you.
There is disagreement between the pilot group and management about how many pilots can be pay protected. The pilots feel that any time a junior pilot gets a bid you should hold by seniority, then the company must pay. Management feels that they should only have to pay protect the number of pilots who were bypassed for an opening.
My suggestion would be to immediately bid what you want to hold, all the way up to the CRJ700 Capt slot. If you find yourself in a good place and do not want to give up your aircraft, or schedule, you can always pull out your bid and select something else.
I was held on the E120 for six months and got paid for the CRJ for five of those months. If you fly your butt off, you can make around $40K the second year and $45K the third. (Counting per diem)
Right now upgrade to CRJ Capt in DFW is about three years and four for ATL. The ATR is about 2 and 1/2 for ATL. Either aircraft pays the same. I'm guessing between $53 and $55K on reserve for a third year CA.
Best of luck!