I used 35 Qcredits to upgrade Y to J for LHR - CMB one way, which I thought was pretty good. Have a look at your calculator and see what stuff costs.
NB: It costs less Qcredits to upgrade LHR - CMB as a complete itinerary for example than it would to upgrade two separate legs for LHR - DOH and then DOH - CMB (not sure if that makes sense but hopefully you'll get what I mean if you have a play with the calculator).