We pretty much knew that there wouldn’t be any real issues with the Mango applications working on the Apollo update. It has now been confirmed with a job posting, which Microsoft wants a Senior Design Engineer in Testing and it states that:
Automated testing of marketplace applications written for Mango, but running on Apollo.
This proves that Microsoft will make sure that the applications that currently run on the Mango powered phones, will also run on the Windows Phone 8 / Apollo update. This is good as we weren’t 100% sure that this would be the case and so it’s good that Microsoft unofficially announced that they will work on the next generation operating system.