I think this action should work. Try it on a selection of drafts one by one and check the results. Revert using version history if you find an issue. Once you are satisfied it works as expected, select all the drafts you want it to run against and run the action to batch process all of the selected drafts,
The action uses a single script step with the following code.
let astrTitle = draft.title.split(" ");
let strDate = astrTitle;
//Only process if it looks like we have a date
if ((strDate.match(/\//g) || ).length == 2)
//Build the new date
let astrDate = strDate.split("/");
let strDay = ("0" + astrDate).slice(-2);
let strMonth = ("0" + astrDate).slice(-2);
let strYear = ("20" + astrDate).slice(-4);
//Rebuild and replace the draft title
let astrContent = draft.lines;
astrTitle = strYear + "-" + strMonth + "-" + strDay;
astrContent = astrTitle.join(" ");
draft.content = astrContent.join("\n");
This should work on Mac, iPad and iPhone, though the way you run an action against a batch of drafts can vary a little by platform and if you are using pointing devices, keyboard access, etc. But you could certainly run this action against all of your batch of 300 drafts in one shot rather than selecting each one at a time to run it.
See Select & Operations.
Hope that helps.