Acquired drug tolerance is by definition completely reversible. The rate at which this occurs is dependent on the particular drug being used, as well as the dosage and frequency of use. If one picks up again after a period of abstinence and goes from, for example, getting high from 10mg of oxycodone to only getting high from 80+mg in two days, they simply did not give their brain enough time to heal. How can any of you claim to know this is untrue when no one participating in this discussion has ever quit for a substantial amount of time?
Also, drug tolerance (I.e. the subject of this thread) exclusively refers to the phenomena of requiring more drug to achieve equivalent effects. It has nothing to do with learned behaviors, patterns, feelings or expectations, although given enough time any of these can be changed as well. What cannot be readily reversed is memory; once you try heroin it is extremely difficult, if not impossible, to completely forget how it feels.
I would certainly welcome hearing about the experience of someone who has managed to ever quit using for a truly lengthy period of time before returning to use. Until then, I will continue to believe what I was taught.