Do you have evidence that Stalin played a role in changing any foreign flags? We can say for certain that he had nothing to do with the Communist Romanian flag in your link, as he had been dead 13 years by the time Romania added the red star to the nation's coat-of-arms featured on the flag.

As far as I can remember, I have read nothing about the evolution of the Eastern Bloc's flags, though I can say that it is unlikely that the Soviet Union would involve itself directly in this evolution. A common misperception of the Eastern Bloc holds that these nations were nothing but the puppets of Soviet policy, and that, really, all national policy reflected decisions made in Moscow. This was not at all the case. I talk more about this here.

In short, without evidence showing otherwise, there is no reason to assume that Stalin played any role in changing any of the flags. These were national decisions, and the Czechoslovaks and the Poles chose not to change their flags.