Board Thread:Plants vs. Zombies 2: It's About Time/@comment-24508466-20141102232720/@comment-25660457-20141204122421

Mr. Boss-inator, I know this problem well. This occurs when the program detects the apk signature is different from any apk available in Google Play. This will happen if you try to install an apk from other sources other than Google Play, so if you still wish to use the apk, I suggest you not to change your .obb file from older versions and use it, the program should have no problem with it, and start to download the new contents for 3.1.1 obb.

Another way to fix this is to use an alternative apk (which I don't think there is another 3.1.1 apk other than from Google Play.)

If all fails, use this method (ROOTED DEVICES REQUIRED.):

Install Lucky Patcher and locate PvZ 2 apk, choose the remove license verification  to fix this problem and wiat for the progress, once finsihed you should be able to open the pak smoothly.