You could not get any seat assignments until the day of departure. By that time, if all of E- is full, they will allocate you an E+ seat, but you are taking a risk that all of e- will not be full. If there is one seat in E- when you check in, you'll probably get that. If you want to secure E+, but a YBMHEU fare, or buy an EPA pass.

DEN-HDN is only 141 miles as the crow flies... does E+ really matter on such a short hop?