Assuming that the financials add up, and only the BE management will know that at the present, this deal seems to offer them quite a lot of options and flexibility over the next few years.
The E175 is a great aircraft to fly on, but it can't be made to work on their wider network and they needed out of the previous commitments, so whether we like it or not, that was a pretty big box they needed to tick.
The release says they take on 24 additional Q400's, at an average of one per month, between March 2015 and 2017, whilst 21 are due to leave over a four year period. That suggests roughly an extra 13 Q400's will be active by March 2017. There are still a few active E195's to be replaced, so perhaps a minimum net increase of 8, over 2 years from March 2015, or one every three months, on average. The BE release says they will become the biggest Q400 operator in the World, and to do this, they need to add a net 8, to overhaul Horizon, so the numbers more or less add up.
However, they presumably have the possibility to extend the leases, or purchase outright, the departing frames, which they have just done with the five bought for the LCY expansion, so they could actually grow the Q400 fleet by anything between 3 and 24 frames, over the next four years depending on circumstances and opportunities?