SFO-HKG: switch business/econ seats with my mom?
 

I have a business class ticket and my mom has an economy ticket on the same CX flight for SFO-HKG. Once we board the plane, can I switch seats with my mom so that she can sit in business and I in economy?

Yes it should not be a problem. Perhaps just inform one of the FAs that you're swapping so they can update their manifest so they're not "surprised" to find a Mrs XYZ in the seat instead of Mr XYZ.

But what you can't do is keep switching backwards and forwards so that you both, in effect, get business class service / drinks etc.

Correct. Just inform the SP when you board. You cannot keep switching. If the crew allows, perhaps you can switch once more.

Like the others, I have never experienced any problem doing this (I have downgraded myself from J to Y on CX, granted short-haul only, to sit with colleagues a number of times). I usually board the plane, then tell someone senior what I would like to do. I always recommend finding the ISM and telling them (or at least the senior purser in the cabin), junior FAs in J class on CX can be a little aloof.