fholbert
Mod's - Please don't edit my posts!
Used to be, flying piston you had no choice but to fly in uncontrolled airspace, at least in western states. Lot of it in NM, NV and AZ extended to 14,500. If you stayed on the airways it was controlled but between airways it was uncontrolled. As an ATC'er we occasionally gave clearances that ended with "While in controlled airspace".Back in my dirtbag 135 freight dog days I worked for an operator that had the ops spec for enroute class g to do exactly this in a Chieftain.
Took my instrument written in '78, there was a question about changing altitudes while in uncontrolled airspace. The correct answer was; You must return to your assigned altitude before reentering controlled airspace.
Other than the 700'/1200' AGL stuff, I don't think you'll find uncontrolled airspace in the lower 48 anymore. It was still in AZ and NM extending to 14,500' as late as 2005.