If you are using a Table Sink, and you change the table name, and in the process you edit so that it matches the name of another table, the UI grabs that other table's schema and wipes yours out.
So, if you have a Table "Table", (with two columns) and a Table "Table3" (with twenty columns), if you want to change "Table3" to "Table4", don't:
change "Table3" -> "Table" -> "Table4" as when you are "Table" your twenty columns will get wiped out.
Instead, right now you need to:
change "Table3" -> "Table34" -> "Table4" (assuming "Table34" and "Table4" are both non-existent) and the current output schema will be preserved.