I'm pretty sure they will modify the taxiways, but I think that'll only be more beneficial for the A380 and possibly 747.
If I recall correctly, the 773 has a longer wheelbase (sorry for that extremely untechnical term!!) than either the 747 or 380, so there would be a greater issue with manoeuverability of the former than either of the "Jumbos".
747s have been regular visitors to BHX for nearly 3 decades, the dearth of the type in recent years has more to do with the economics of operating big twins than an unwillingness of operators to put them through BHX.
Diversions are now also rare occurences because of Cat IIIB operations at most major airports.
Were a carrier to decide to opperate regular services with the A380 I feel confident that the handling agents would ensure the appropriate equipment would be available, but exactly who is going invest in such equipment "just in case"? Certainly not commercial operations that expect / require to turn a profit.