The Grape Jelly trick is a spoiler/workaround/trick which is no longer necessary in 5.1 and later versions. In 4.87, when your intelligence and constitution changed, your mana and hit points did not change. Thus, in order to get the benefit of the increased values, you have to let a grape jelly (or other creature, but grape jellies are otherwise harmless) drain you to a low level, and then drink restore life levels potions to go back up with the increased stats.