There are 3 aircraft types that will be leaving the Skywest fleet and they are the A320( gone with 12-18 months), F50 (within 2 years if not sooner) and the F100 (gone with 4 years if not sooner).
The ATR will not only increase in numbers on the East coast they will also replace the F50's on the West coast. Ejet's will be replace the F100's and who flies them will depend how the EBAs at VA and XR work out. The 737's will replace the A320(s).