Bedrock generating instead stone and other blocks under y=0

I created a world on the beta and left the beta after glow lichens and I was wondering was added. When I joined back today, all blocks from the same world from y=0 to y=-64 were bedrock, but on worlds I created today they had stone and ores and other blocks instead of  the bedrock on my previous world. I know that the bedrock shouldn't be there on the previous world so I was wondering if there was anything I could do to fix it.