Thanks for responses.
The tickets are for a couple of weeks time and the flights are already priced high - so the cheapest CW return is already north of £6k. It’s a business trip so I am not paying as such but at the same time want to keep the price as low as I can.

Also on the way back I don’t know if I will need fly back from NYC at 6pm or 11pm or indeed may need to fly back from Chicago rather than NY so wanted the flexibility to change if necessary.

A fully flexible J class return leg is pricing at circa £4.2k and the cheapest return leg with £300 change fee is pricing somewhere between £3,600 and £4,600. So on a couple of the flights already the fully flexible J fare is already cheaper than the cheapest “lowest” CW fare. So I thought I might book myself on a flight at 9pm and then could change earlier or later or even back fromChicago with minimum cost if I booked the return leg at £4.2k which would be at most a £600 extra cost now rather than waiting another 10 days for my plans to be fixed by which time the return flights may all be in excess of £4.2k.

Anyway my real question was that if I wanted to do this how I could go about booking this since I called BA and they told me I couldn’t combine a R or D class fare on the way out with a J fare on the way back but I may have just got a useless agent!