If you have checked luggage, I would be careful with switching to UA.
It seems that CX has tightened its generous through-check policy.
https://www.cathaypacific.com/cx/en_...QUESTIONLIST_2
It depends on the agreement between CX and UA. You probably will have to clear immigration, collect your bags and check them at UA counter. If you CX flight is delayed that will be iffy.

If you have already purchased your CX ticket, fares are usually cheaper to SGN than to HKG. It might need repricing if you have checked bags.