The guidance for MoT inspectors on this point states that the headlamps must be 'matched', which is described as: "emit light of substantially the same colour and intensity, and are the same size and shape that they are symmetrical to each other", which means it is something of a grey area. It is possible that it will pass, but you should prepare for the eventuality that it will fail on this point and so you may wish to swap the older bulb to the same type.