Unfortunately most of the story is password protected and I can't find details elsewhere.
WestJet details 737 premium-economy retrofits | Aircraft Interiors content from ATWOnline
WestJet’s premium-economy trial has paid off, convincing the Calgary-based LCC to launch a fleet-wide retrofit that will see new, larger two-by-two seats replacing the current three-by-three test configuration that uses a blocked middle seat. The Canadian carrier plans to start the modification work on its Boeing 737 fleet late this year and expects it to take about 18 months.