@Tim_Meske Several things…
You keep noting “the far left column”. Column position has no particular significance; AppSheet identifies columns by name (the column header) and ascribes significance by name. That the column is “the far left” doesn’t mean anything in particular to AppSheet.
The [_RowNumber] column is defined and maintained automatically by AppSheet, and used as the KEY column if AppSheet cannot find a more suitable KEY column when the table is created. The values in the [_RowNumber] column will always be unique, but may change if you re-order or remove rows in the spreadsheet. For this reason, the [_RowNumber] column is considered “unstable”, and not an ideal choice for the KEY column.
You can change your KEY column at any time. But keep in mind that the KEY column values are how Ref values refer to the table’s rows. If a key value is stored in a Ref column, that stored value is not automatically updated if you change the KEY column. As such, that stored key value will likely not match a value in the new KEY column.
For instance, suppose the old key column used numbers (1, 2, 3, …) for key values, but the new key column uses letters (A, B, C, …). A Ref column that contains an old key column value (e.g. 2) won’t match any values in the new key column, breaking the reference.
If you delete all the rows in your spreadsheet, you will eliminate any concerns about duplicate or broken keys.
I’d like you to spend some time tinkering with keys and references to better understand them. They are extremely important,and understanding them will benefit you greatly.