First, I dont know does the "permanent tolerance" even exists as the official term. I do not it does. But from (not just my) experience, when you reach a certain level of tolerance, lets say 100mg of morphine daily just not to be in withdrawal and need 150mg to feel high, and you stop taking opioids for 3 months, your tolerance goes to baseline (after the inconvinience of withdrawals). Then, if you start taking morphine after 3 months you will have a window of few days (week or two max.) when you will need much less morphine to get the desired effect, but your body will adapt much faster than it adapted first time you were taking opioids. In the matter of a week , or so, you will again need, in this scenario, 100mg of morphine just not to be in withdrawal again.
I dont know how much is this true for SNRI effects, but I have found it to be true for other classes of drugs (like benzodiazepines).