Does the PAYS model envisage potential retrofitting of batteries on buses to convert them from diesel to electric?

No. A new electric bus is more cost effective than buying a diesel bus and then tearing out its engine and tank to make a custom retrofit for that particular chassis, with little or no warranty coverage. While there are a handful of retrofit electric bus producers, the supply chains for retrofit conversion buses are difficult to develop at high volume, especially on the scale that new buses are already being added to the global fleet every year.