These displays have unused pads on them including space for an 8-pin voltage convertor chip. This is designed to allow a 3v version of the display to be sold by adding a negative voltage generator to the LCD module.
Converting the module to 3v is simply a matter of soldering the correct voltage converter chip (ICL7660) onto the space provided, plus two 10µF capacitors across pads marked C1 and C2. Finally, the voltage convertor needs to be enabled by soldering across the jumper J3 and removing the solder from across J1.
The ICL7660 voltage convertor can be obtained for around £1. Finding the correct capacitors in small quantities was a bit more tricky. Unless you have access to some already, you'll probably have to buy 100. Here are some suitable ones.
It can be seen that the voltage converter and additional capacitors are surface-mounted. Although they are more fiddly to solder than "through the hole" components, a normal soldering iron is fine (preferably with a fine tip). The technique I used is to melt some solder onto one of the pads for the 7660 chip then, holding the chip down against the pads, apply the soldering iron briefly to the relevant leg of the chip to remelt it and "fix" the chip in position. At this stage it is important to ensure that the chip is correctly aligned with all of the pads. If not, repeat this procedure until it is. Now, the other 7 legs of the chip can be soldered in place by heating each leg in turn and then dabbing the solder onto the leg. Miraculously, the solder will flow onto the pad and over the leg! Once you have soldered all 8 legs, check that there are no "bridges" between adjacent legs. If there are, a quick dab with the soldering iron should sort it out - if not, hold some solder wick between the affected bridge and the soldering iron and that should suck up the excess.
Now use a similar process for each of the capacitors; melt some solder onto one of the pads, hold the capacitor in place on top of it, then reapply the soldering iron to melt it into position. Check the alignment, and repeat.
To remove the solder from J1, heat up the pad with the soldering iron and either use a "solder sucker" or a piece of solder wick until there is a clear gap between each half of the pad J1. Then heat up the J3 pad and apply some solder to bridge the gap.
The picture below shows the new components in place as well as the change in jumper position. You'll also notice that I have soldered a 16-way socket onto the back of the LCD circuit board. I find that this makes it easy to connect serial/parallel convertor boards if required.