Hi Gone2TheDogs,
The “Single Line of Text” is only some characters. There is no validation. But the “People or Group” is a type to put domain
account. If the characters is not a format of domain account or the user is not exist, the validate will be fail. So , in SharePoint 2007 you can’t convert “Single Line of Text” to “People or Group” once the column was created.
If you really want “People or Group”, I suggest you to create a new column, this column type is “People or Group”.
Thanks,
Jack