I broke in my nitro engine using 20% nitro. Later on I used 30%. Would it hurt to drop back down to 20%? I heard somewhere that it would. Any advise would be greatly appreciated.

no it won't hurt it, but you'll have to retune your motor

I think it really depends on the 30% you were running and the 20% that you switch too. If you were using 30% with 8% oil and switch to 20% with 16% oil don't be surprised if you eat glowplugs. Your engine will be looser once you've ran the 30% so when you switch back you need to run it a bit rich and that causes the plugs to go.