Nothing I love more than shopping out a tech the turn before my Wonder finishes.
It's great fun, but I prefer doing it the turn
after I've finished the Wonder. I've been bitten on the **** a couple of times by selling the tech just one turn earlier. (I wonder, are you earning more money by selling the tech when the Wonder hasn't been built yet? It would make sense).
It's only happened a few times, but sometimes the AI manages to immediately switch to e.g. JS Bach's, and finish it on the inter-turn, leaving me high and dry! This is maddening. Possible explanations:
a) The AI happens to be 1 turn away from a Wonder that costs the same.
b) The AI is over production for JS Bach's, because building a more expensive Wonder, but decides JS Bach's is a better bet (which it almost certainly is!)
c) I think I read somewhere here that AI civs can use Military Great Leaders to hurry anything, including
Great Wonders.
c above is something you just can't fight against; but I keep a close eye on the other possibilities by using the Investigate City screen whenever I get the "Russians are building Wonder X" message, and recording the results so that I don't have to Investigate City again for a while (spreadsheet attached to post #10 in
this thread). You also get great info on the enemy's resource, luxuries, science rate and typical size of city defence from this screen. With this information and sometimes with a hunch, I sometimes feel safe to sell a tech just before I complete the Wonder, like Hellfiredoom.