I don’t know if it’s having been already mentioned here, but I found a method to merge your layers apart from the base layer.
In 2 steps:
you must undo the current top layer (lets call it Layer 2) that you don’t want to merge with the base layer (Layer 1)
then Hold Press to redo this layer (Layer 2).
And from now:
if you trigger an overdub (Layer 3) before the current EOL , this new layer is merging with Layer 2 and not with the base layer (Layer1).
but if you trigger your overdub after the current EOL, your layer 2 will be merged with layer 1
In the first case, with this technique you can merge as many layers as you want apart from the base layer. So you can let your base layer with its original content.