Kiwi Tyrant
Evil Jollification
I've found if you start a National Wonder (thus initially meeting the 'building in every city' requirement) and then place (or take) new cities afterward's, you are still allowed to carry on constructing the Wonder. If you change building it and then try click back on it, the option is then grayed out. In the example pic, I have exploited this by getting the Wonder underway just before starting a multi-city expansion....
I feel you should be able to keep the amount of production you have put into the Wonder, but it should be immediately be put on hold until the requirement's are established again.

I feel you should be able to keep the amount of production you have put into the Wonder, but it should be immediately be put on hold until the requirement's are established again.
