how could I swap the symbol?

hi all~

I have a main symbol call "Layer1" with a timeline montion，

than include the"symbolA".

so I duplicate the symbol "Layer1" to "Layer2", then I want to replace the "symbolA" to "symbolB", and keep the timeline montion..

but there's no menu option to do that, but for the elements(image) can be, like the "chage the image resouce" option..

I hope you can understand what I am saying.

I want to keep the timeline montion and swap the symbols.

thanks.