As Andrew mentions fast change or conditional show are not features of Qlik Sense which is aimed more at self-service visualisation. The capabilities in Qlik Sense make it very easy to convert existing charts to differing formats, simply by dragging and dropping the required visualisation on top of the chart. In this way it is very easy for the user to create a custom view of Base Sheets, building their own analysis and sharing back this analysis to other users.
If you MUST have 1-click to switch between a bar chart and pie chart put the pie chart on the next page and use the '>' next page icon.
I hope this helps.