For current 7" devices nothing will change, after you update these to Windows 10 you'll keep your desktop, thereby preserving continuum.
The confirmation comes after Microsoft told Neowin that Windows 10 would come to small ARM tablets with displays of up to 8 inches. Belfiore's comments also clear up any confusion about whether the desktop would remain on ARM devices, which can't run traditional Windows applications.Source: Neowin
Since ARM devices won't exceed 8 inches, it means they won't have the desktop. ARM processors couldn't run traditional Windows applications regardless, unless the software was recompiled or written for the low-power chips.