The 50 seat scope at United has a max weight of 65,000lbs, so they have to restrict 10,000lbs off of the CRJ700 and certify it as a CRJ550. The E-170 weighs 85,000lbs (someone who flies it can give a more precise number here), so in order to make it a 50 seat aircraft, you’d be bringing down the max certified weight down by 20,000lbs. The E175 is already restricted by 3000lbs to meet scope, if you wanted to make the E175 a 50 seat aircraft, you’d have to bring the weight down by an additional 21,000lbs.