Hi, I'm curious to know when/how it updated previous selections bc that isn't the typical behavior. When you make an edit to a custom form field like that, it applies moving forward, not retroactively (unless you change the nature of the field like if you remove an option from the dropdown altogether, it will no longer show on other records since it is completely removed). The only time something can retroactively update an answer is a calculated field and you choose to apply the new formula to existing calculations. My recommendation is to grab the ones you want to update in a report and bulk edit them, like it sounds like you've done.
If this helped you, please mark correct to help others : )