I know we can safely go up in nitro %, and just retune and do another break-in, but can you go back to the lower %? Or has the sleeve/piston taken a set to the new higher fuel content?

Only the facts, please.

Your not suppost to go from a higher to a lower I mean I guess you could but you engine might not run right

Not to be smart-alecky, but please, just the facts on the subject, not guesses or opinions. If someone has the true technical explanation of this question, please reply.
Thanks.

It doesn't matter what percentage you use or change to. The thing that matters is that the engine is shimmed correctly. Your engines instructions generally tell you what it's shimmed for. Going up in percentage will make you have to add a shim, while going down will make you take one out. Having less compression, going from 30% o 20% will not harm the engine. Having it shimmed for 20% and going to 30% might cause higher temps and blown plugs.

You don't have to "re-break" in the engine, but remember to retune. Just richening it up so you don't run too lean is all you need to do, and retuning for the new fuel.

Derek, Thank you for the facts.
Dave