The place to start would be to figure out how big a turbine the existing towers could support. If you look at the towers for big turbines, they are big tubes of steel, I guess the cylindrical shape is important (strong). If you look at really big power poles, they are lattices (not as strong).

There are power poles that are sort of in between, roundish with flat sides, but they are usually more medium sized.

So you quickly get to asking why do utility companies put their money towards building large (which helps with efficiency) turbines on the best wind sites, rather than smaller turbines on random sites that they own.