According to an Intel Product Change Notification 112980-01 Intel will change the Baytrail Celeron processor used in the NUC model DN2820FYKH from N2820 to N2830. The product will still be called DN2820FYKH, so it will be slightly challenging to determine whether you're getting the new or the old model when ordering online.
If you're shopping in person, there is a way to differentiate the new and old model though. There is an SA number in the bottom of the box. If it says Version # H22962-103 it will have the new N2830 processor. The models -100, -101 and -102 are equipped with the older N2820 processor. Alternatively, the AA number can be used to check the model. AA number H24582-203 is the new N2830 model whereas -200, -201 and -202 are older models. Messy, huh?
Anyway, the old DN2820FYKH has been out of stock at most retailers for a few months already. The official story is that Intel underestimated the demand for the Baytrail NUC. It might be that when the DN2820FYKH becomes available again, it'll be all N2830 processor versions that will be available. We'll see.
So what's the difference between N2820 and N2830 you ask. Well, not that much, but there are some. The clock speed has been bumped up by an insignificant amount from 2.13 GHz to 2.16 GHz. The N2830 will be able to support DDR3L-1333 memory instead of the DDR3L-1066. Perhaps most significantly, the N2830 does include support for Intel Quick Sync Video. The Quick Sync Video will enable hardware encoding of various video formats, which should improve the performance significantly if you're in need of video transcoding.
You can also have a look at the differences between N2820 and N2830 on the Intel Ark website.