Revalidate and Save of a Data instance is not required is the understanding we had while building this tool. If you have a scenario where revalidation is required for Data instance please let us know, so that we can include even Data instances into the selection.
If, you are looking only for the implementation logic so that you can use it for your application specific requirements then you have posted the question in a wrong forum.
You can ask any clarifications or queries around the Upgrade tools here or give your feedback here for those tools. We do not expect asking how a feature is implemented in this forum space.
Dropdown list is getting populated in 'Revalidate and Save ' tool as per your Application rulesets.Some times this ruleset drop down wont display all the rulesets because your application contains more than 10 rulesets inclusing built on applcations.
So try to create separate application with less number of rulesets to see the desired ruleset in the drodown