You will get some airplane replacment with the E175's. But all of them. Upgrades are a combo of growth, and attrition. Even with a even swap the E175 requires more crews per plane then the CRJ. Doubt SkyWest ever get the E2. (But concert them to regurlar 175's) All these "E/Max" planes are a marketing scam. Engines weigh more, landing gear have to weigh more, and the extra MX intervals and just the markup on the sticker cost alone done make up what unproven fuel efficiency they promise...