No offense taken. When you change the nitro content of the fuel, you change the operating temperature characteristics of the engine. This can cause the internals some problems from the standpoint of how the various materials react to the new operating temperatures. If you just throw it in and go and do not allow the engine a chance to get used to the new stuff, you take a chance that the engine might break. How exactly will be determined by the materials that make up the guts of your engine.
When I said more agressive, I meant a longer "break in" period. Instead of one tank to allow the engine to get used to the new fuel, you might want to run a couple of tanks in "break in" mode.
Everything else you said is accurate. Weather changes, particularly dramatic temperature and humidity changes, will in most cases require a change in needle settings. Heck even changes in location with respect to altitude will require needle tweaks for best performance. Changing fuel brands and/or nitro content will require a needle tweak or full retune depending on which route you take. But the initial break in is for the purpose of, as you put it, mating the piston and sleeve. It is also prepping the engine for its intended use with respect to allowing the internals to handle the temps and getting the rest of the internals working together. That is why getting up to a good operating temperature during break in is sooooo important.
Even after that intial break in any kind of thermal shock to the engine can lead to warped parts. That thermal shock can be from cooling down too rapidly or by getting a sudden change in operating temps from say a different nitro content.
The bottom line in all of this is to take care of your engine so it lasts longer. Can you just slap in the new fuel tweak the needles and then turn and burn? Sure. Is it the best thing for the engine? In most cases, NO. Do what you can afford and feel is right.
I can't remember who posted it and can't seem to track it down in a search of this site, but someone posted a great write up found in a mag about break-in processes and changing fuels. It was very thorough and a great read. I'll keep looking, and post it if I come across it...in the meantime, if someone else finds it post it here.
Found it.
http://www.rcnitro.com/articles/nitro_fuel.asp
Check about two thirds of the way down in the section titled " Can you change the amount of nitro in fuel without damaging the engine?" This guy is an engineer like me, only he is published and says it better.
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