I have the same problem.
I bought the program from Appstore on 2/2/2015, everything was good until today. It said that the program wa damaged and that it has to be replaced by downloading it again from th app store, but I can't do it cause it won't let me do that because of the os.

I still use Snow Leopard and I don't wanna/can't upgrade my os in my computer. I need to be able to download an older version.
Don't leave us alone like this!!!