Your piercings can shrink back down after stretching them, as long as you do not stretch them too fast. If you stretch your piercing to fast you can cause a tear in your piercing that will develop scar tissue. The key to keeping a happy piercing is to stretch slowly.
Body Jewelry By The Chain Gang has Double Ended Stretching Crescents which you see to the left and are great to very slowly stretch to another gauge. It is a slow process, but you will not tear your piercing either which other then the obvious, will also help you if you should ever want to let your piercing shrink down some.
Once you have stretched the piercing out some using the Crescents or Weights, then you can use the Insertion Tapers to insert your new larger body jewelry.
Another way to help stretch safely is to use additional weight. If you like to use weights to stretch your piercings make sure you do not use a really heavy weight, because it can cause your piercing to disfigure and this will prevent it from healing back to normal.
Another thing to factor in is how big did you stretch you piercings. Most piercings that are no larger than a 0 Gauge normally do go back down as long as you did not cause any scar tissue during the stretching. This process may take a while but there is hope. Over the 00 gauge, may shrink down some, but it is unlikely that they will ever go back to where they started.
And last but no least, how old is the piercing? If it has been several years then it may not go back either. Stretching is something to take very seriously as you may have these holes that you made for the rest of your life.
My best advice is to stretch slow and do not do this "just because". Think about it just in case it is not what you want to have for the rest of your life. I always look at anything like this as if it was permanent, that way you are sure when you make your decision.