How to enable 9-slice mode in an already finished Illustrator symbol?

Question

Design

When you create a symbol, you can immediately check the box "Enable guides for 9-slice editing".
And what to do if the symbol is created without this checkbox - how does it enable 9-slice?

Answer 1

I can’t check what’s going on with the 9-slice of the finished symbol now, but as an option I can offer this solution: copy the entire contents of the symbol into a new symbol (already with 9-slice), select all copies of the old symbol in the symbol panel (Select all instances in the side menu), select a new symbol in the panel and replace copies of the old symbol with a new one (Replace symbol).