The effect of Hamlet's joking about death and worms is that Hamlet was like suggesting that Claudius will before long die and join Polonius, asking him if he would like for a messenger to be sent to find him in heaven or go to hell himself and seek for Polonius there. By the act of suggesting that he go to hell to find him is like saying that he shall soon join him and/or that Hamlet believes he actually belongs there with his counsellor.