It has been rumored for quite sometime that current BlackBerry users will not be able to upgrade their device when BlackBerry 10 OS comes later this year. And this rumor was confirmed by RIM’s CEO Thorsten Heins, according to The Verge. David Pierce wrote:

RIM CEO Thorsten Heins confirmed once again this morning that the company has no upgrade path for existing BlackBerry 7 devices. In fact, the two seemed destined to co-exist: Heins spoke about the new BlackBerry 9220 (and confirmed the existence of the 9320), using their "successful launch" as an example of how RIM is still "onboarding people from featurephones to smartphones," a process that will take the next two years or so. The company seems to see BlackBerry 7 as the OS for its entry-level devices, while BlackBerry 10 will occupy the higher end of the company's range. RIM has promised to upgrade existing PlayBook hardware.

So in short, if you are planning to buy a BlackBerry handset it’s better to wait till the end of this year and see what BB 10 has to offer, rather than regret buying an out dated handset that can’t be updated with the new OS six months down the line.