To put some numbers on this, the difference between Q and H on (for example) JFK-LHR in March is around USD275 each way, plus a USD275 change fee.

You'd then be faced with an additional fuel surcharge and govt tax for the upgrade.

In all, probably not worth it this time around, but it is worth getting to the airport early and asking about cash upgrades. ISTR numbers of around USD199 for WT to WT+.

hth