Upgrading from "J" to "A", is it possible?
 

J2RTCX CX J Round-Trip 56500 (HKD)
A2RTCX CX A Round-Trip 58730 (HKD)

I have a rtn ticket to LHR which my company issued through a travel agent, booked in full fare J, can I call up CX ticketing, give them the ticket number and pay the extra HK$2,730 for First Class in A?

The answer is probably NO. You have to call the TA and get him to do this. But it is often the case that your company/TA get a negociated rate, so that you get the flexibility of J but the company pays somewhat less than full fare. Also, in most company/TA contracts, the TA should ask the company for permission. So it all depends on the TA as you are asking him/her to do some complicated accounting.