That is false. Only semi-autos that accept detachable mags, or have fixed mags that accept more than 10 rounds of ammo, are regulated by the AW laws.
You may use uppers with pre-ban features on the DPMS single-shot lower, or better, on the GoGoGadget fixed 10-round lower. You may also install a folding or collapsable stock.
HOWEVER, a new, recent, little-known law made any rifle under 30" in total length an "assault weapon", even a single-shot. And, unlike the Feds, the PRK measurement is done with the stock in the folded/collapsed position, NOT in the open/extended position. So, if you install a collapsable stock, make darn sure your upper is long enough to get you out beyond 30" with the stock collapsed.
Note that Kalifonistan's laws have even turned some LEVER-ACTION COWBOY RIFLES into "assault weapons", due to the fact that they have magazines that accept more than 10 rounds. Lots of people don't understand this, and didn't register...
-Troy