First, THEORY DOES NOT MEAN WHAT YOU THINK IT MEANS!
"There are 9 planets in our solar system. No wait, 8, no 9. (I can't remember if Pluto & it's moons count today or not)."
This is a matter of classification based on less complete knowledge. As our knowledge becomes more complete (often with better detection methods in fields like astronomy) our classification of particular objects may change. The discovery of more pluto-like bodies is what "demoted" pluto. There are 8 regular planets, and pluto et al are "dwarf planets".
This principle is true of biological taxonomy as well, given we have genetic information to go on now.
"A rat has recently been found that was thought by scientists to be extinct for the past 11 MILLION YEARS"
There was no evidence of it being alive, therefore we thought it was extinct. We did not know it was alive until we found it. So what?
"Drinking a glass of wine each day prevents cancer. Next day: Drinking a glass of wine has no effect. Next day, Drinking a glass of wine each day will kill you."
This is a matter of which study you look at more than scientists having a change of heart. There's always chance in there since the people in each study will be different. Study methodology may be different, especially if the scientists are working independently, which can on occasion mess with the results. In fact, this, though confusing for the laymen, is a sign that science is working as it should. Different scientists are getting different results, then they'll be examining each others' studies closely, see if they can pick out what might be wrong, try to copy each other and get the same results, try different methods, etc.
Besides, this means you're going on what the news is telling you, which you should never, EVER do. The vast majority of news people (particularly TV news people) don't have any more of a clue about science than you do. Not to mention they are often years out of date on the studies, so far that undergraduate biology students study -in their textbooks- what the news reports on.
