Whoopee Doopee, we have fun!
You lost the game!
In the case of building over a Coal Mine or Iron Works, you already have the presence of the industry.
Question: Can you use any card to perform the build over action or must one use a Coal Mine for a Coal Mine and a Iron Works for an Iron Works (presumably a proper location card would work...).
This came up in a game near the end - my claiming that I only needed to play a card (small 'c') - but I yielded to the group who demanded play of the corresponding industry card. Rule book was not specific on this point as far as I could find.
It was only going to be a 1 point difference in outcome. Scores were 122, two of us at 117 (including me) and one at 115!
It is a great game.
When you're building over a tile you are still building an industry, and so are subject to the normal rules for building an industry. Also note that the technology number of the new industry must be greater than that of the tile you are building over.