IT is also true for ore - you've just been lucky.
The chance of change is small, for each time resources are harvested - so pretty much random. It could occur with the next respawn, or take 10, 20 or more.
There are only two, linked, reasons for exploring and mapping for resources, now - QUANTITY & QUALITY.
While the TYPE of ore or wood now changes at random intervals (change chance after each harvesting of the available resource), the AMOUNT the resource square gives on average doesn't change. Similarly, each resource square apparently has set ratios of "colored vs. normal" whose level stays at the same chance, even if the resource type changes (not sure if this is for both wood & ore, or if each have their own separate value).
So, if you have a tree that produces 50 chops of wood (500 or 550 logs, depending if you're elf or human) on average, it will always produce that average amount if it has recovered from the previous visit, regardless of the wood type. If a spot usually gives 25-30 ore, that will stay constant too.
If a spot produced 70% (on average) colored resources, on every visit, prior to the randomization changes, it will STILL produce that ratio, any time that the site is giving colored resources.
So, if the previous example of a mining spot was 25-30 ore, with 70% colored, and was a bronze spot before the changes, it STILL puts out 25-30 ore, and 70% of it will be colored ore, be it Dull Copper or Valorite, so long as it is currently capable of putting out colored ore (as even if it is Iron, you can raise it to Dull Copper with a prospectors' tool).
Logging, in my experience, has been the same, except there's no way to raise a normal wood resource square to oak. But, if while exploring, I found an oak tree that was putting out over 50% oak, and allowed 50 chops, I'd mark it in a heartbeat, and chop it every time, hoping it changes to one of the good woods from my efforts later.
The exploration now is to find the high-yield places with good colored-to-normal ratios. Your haul from them will still be random, but you'll get a lot more quality wood over time that way as the high-output trees change to things you want.
I'm already using this strategy for the rare times I still book mine. I eliminated all the under-10-ore spots, and all the "less than 50% colored" spots from my two Trammel/Malas ore book sets long before the change. Now, I have places that, while random (and I can tweak a bit with the PT & GPA), give me a LOT (comparable to Fel mining) of colored ore with a minimum of stops.
