Old nitro is no more prone to explosions than new. But if it was not stored in a tightly sealed container it could have absorbed moisture and thus not burn as well and coud cause corosion if left in the engine.
Having said that, as long as it was stored properly it will keep for a long time. A year at least.
As to the 30% vs 20% issue.
You will have to retune and you may notice that the engine lacks some of it's former power and it may run a bit warmer than with the 30% fuel. It should not be a huge difference though and retuning should should get you close to where you were, performance wise.