Yes, a group field is a repeater with a single row. However, you can’t just convert the repeater to a group. The act of switching the repeater to a group in ACF removes the sub fields.
You can probably do this if you export the field group to json, then modify the field information in the json file to match a group field and then sync the field group. I would do this by using local JSON https://www.advancedcustomfields.com/resources/local-json/, edit the file that acf creates alter the updated time and then syn the group. The important part here will be making sure that you keep the same field/subfield names and field keys.
I’ve never actually tried this, but it should work. But, as always, back up your site and DB before you try it.
