How can I convert my toilet from a single flush push button to a dual flush?

Yes, in principle this will be fine. Just check the outlet at the bottom of the flush valve is the same diameter as your existing valve and it will be a case of simply swaping the two valves. The job will be easier if your new flush button is the same diameter as your old one. I'd suggest installing a new fill valve whilst you've got the cistern apart and use a fresh set of fixing bolts and washers if it's a close coupled cistern.

Rupert
October 2019