WGT adopts the One ball rule, so once you have started a round, you must finish with that exact type of ball. It reverts to the starter ball so you can finish with no advantage.
When you tee off it warns that this is your last ball of that type, would you like to buy more, If you dont at that stage and you lose it, the starter ball comes into play. Personally I would like WGT to add a buy now if you lose that ball at any stage - a win win situation for WGT and the player.
If you had searched the forums you would have found many posts about this.