Hi partner,
If we changed the option values in field settings, all previous data of this field will disappear. So you could refer to the following ways.
1.If the data volume is not very large, you could bulk update the values with excel online.
You could create a new multiple option field with the new right values and put all the old and new multiple option field on the view.
And you just need to copy the value from old to new cause when we edit options in excel, we just need to enter the right label, not value.
Open the entity view and click "Export to Excel"->"Open in Excel Online".

And then we could edit the records like what we did in excel.

2.We still need to create a new multiple option field with right new values. And then we need to create a console app with C# to connect to D365 and make a upgrade with some conditions.
For example, get the old option first, and if old option label equals "Lung Cancer", then set the new option field value to 2001 and just do a loop.
Hope it helps.
Best Regards,
Leo