It is cost efficiency that has improved dramatically in the last 7 years, not device efficiency. Maybe 7 years ago you bought a module with a 14% sunlight-to-electricity conversion efficiency and today the same manufacturer would have a 16% efficient panel in the same part of their lineup. That is not a dramatic change. It's certainly not worth the cost of replacing panels that work just fine with this year's model.

You'd pay 75% less per watt of panel capacity today compared to 7 years ago, due to manufacturing improvements up and down the supply chain plus brutal market competition between manufacturers. That is a dramatic change.