G. -

You need to check if you have a rootrot. Usually the leaves are kind of limp and the rotting lower leaves can have rotted leafstalks and the main stem is rotting too. In this case you need to perform surgery, cut off the crown of the plant, remove almost all outside leaves to last medium size 3 leaves and root it again. It roots in a light soil or water.
Just in case - put a couple of leaves down too.

If it is not a root rot - may be it is just old leaves - their time to go - plus the transplant shock. Remove the lower old leaves and see what happens.

Good luck

irina