Any spring that is harder means that the suspension travels less distance for the same load, which means if your damping was correct before then its underdamped now.
If you just want to make it "phat" then lower it away, if you want it to handle better than decent shocks too would be a wise investment.

Best thing you can do on a budget is buy a good make but low end all in one kit like a spax kit or a koni kit, which has springs and dampers designed to work together.