I don't know about the toyota, but it's generally pretty easy to convert a shyster/yale unit from 36 to 48V.

Is it worth it? That's up to you. Your comment kind of concerns me, in that the battery is part of the counter weight of the truck. If your 36V battery is too small, it will affect the saftey of the unit. There is a minimum weight listed on the number plate, please ensure that you are over that.