That is a common setup. Drop one cell down on the lamp vs battery setup. A 3 cell lamp in a 4 cell light, 4 cell in a 5 cell etc. Some lamps tolerate this better than others. You will get less life from the lamp but a lamp that burns twice as bright lasts half as long, so to speak.
All lamps are over driven. The rating of a 2 cell lamp is 2.4 volts on average. We give it 3 volts. While there is some voltage sag due to current draw the voltage still is over the actual rating of the lamp for the useful life of the lamp.
The percentage to remember here is 50%. Most lamps will flash out if you exceed the voltage by 50%. So the two cell lamp will not work long if at all at 4.5 volts (3 cells). In that case you would almost be doubling (100%) of the lamp's rating.
Go to
Topbulb's website and check out the ratings for many PR based lamps. They have listings for the unmarked Mag lamps. They are simply marked 2 cell, 3 cell and so on.
Basically if you go one cell over than the rated number for the lamp you will be OK. Brighter and better but a little less runtime. Just don't go too big and melt the reflector.