[flexcomponents]What happens when you use a SWC compiled with Flex SDK 2 in a Flex SDK 3 project?

[dorkie dork from dorktown]

I have a few projects that are using SWC's compiled with the Flex SDK 2. I
want to move these to Flex SDK 3 but I don't have access to the source code.
What happens when I want to use these swc's in Flex 3 projects? They seem to
work but what is going on internally?

Thanks!