It changes a single 50% multiplier to a 100% one... (or technically, changes a single variable from not-effective to normally-effective)Only Hypnosis's accuracy was changed (10 points, whoop-de-doo); its core mechanic (inducing sleep) wasn't. Therefore, not a big deal.
According to this guide, one of the core Pokémon game mechanics (type interaction) will change; such a change would intrinsically remove battling compatibility between BW1 and BW2. Type interaction is a heck of a lot more complex than accuracy, my friend.
Also, there's a 99.99% chance that the guide's type chart is erroneous. >.>
It's still likely a typo, but the comparison to the Hypnosis change is entirely valid, since they still would have the exact same problem (what would've hit wouldn't have, what would've been KOed wouldn't have, threatening a de-sync in either case) that could be solved in the exact same way (host's cartridge sets the game mechanics).