According to engadget:
RIM says BlackBerry Enterprise Server 10 users will get legacy device support, IT admins (may) rest easy - Engadget
Corporate server managers everywhere were given a jolt this week when rumors emerged that RIM might implement a hard cutoff for BlackBerry Enterprise Server 10: any devices based on BlackBerry 7 and earlier might not connect at all, leaving IT leads with the uncomfortable choice of either running a BlackBerry server platform that's supposedly without a future (BES 5) or having to upgrade both the servers and phones all at once.
RIM is putting minds at ease -- more or less. The company's Kim Geiger has confirmed in a statement to the media that BES 10 will support legacy devices when it ships in the first quarter of 2013, and that existing server customers will get an upgrade for smooth sailing around when BlackBerry 10 arrives. That's no doubt a comfort, but there are lingering doubts.
Rumor source BGR maintains that companies will have to run both the old and new servers side-by-side to address everyone, which could make a truly harmonious environment complex, expensive or both. We've reached out to RIM for a more definitive explanation to hopefully settle the matter. In the meantime, we wouldn't panic; no one is being pushed to adopt BES 10 right away, and those that want to upgrade don't have to give up their legacy hardware.