The Convert option allows you to swap a chart type e.g. if you have created a Bar chart you could drag the Line Chart widget on top of the Bar Chart and a context menu will appear with options to Replace or Convert.
If you choose the Convert option then your dimensions and measures are maintained in the new chart type.
Hope this helps.
3.11 Converting a visualization
You can convert from one visualization type to another by dragging a chart from the panel on the left-hand side onto the visualization that you want to convert. All properties that the original visualization has are transferred to the new type. The new visualization uses the dimensions, measures, and settings that are applicable to that visualization type, but all properties from the original visualization are saved, even if they are not available or visible in the new visualization. This means that properties can become available again if you decide to convert to yet another visualization type where those properties are used.
Do the following:
1. While editing a sheet, drag a new chart from the panel on the left-hand side onto the visualization that you want to convert. The shortcut menu opens.
2. Select the conversion option. The new visualization is displayed, using the data from the original visualization.
When you convert to a new visualization type, some of the settings from the original visualization may not be optimal for the new visualization, for example, the sorting order. Therefore, you may need to make some changes in the properties panel, so that the new visualization is displayed as you want.
You cannot convert to or from a KPI, a map, or a text & image visualization, nor can you convert a master visualization.
This is an extract taken of the help page of QlikSense https://help.qlik.com/sense/en-us/pdf/Creating%20Visualizations.pdf