the transfer vs copy issue is VERY easily addressed...copy is the best...
copy character...and all its data...but TRANSFER the gold and the non soulbound items from classic...
or prevent copy if there's soulbounds items in the inventory...only allow gold transfer...gold which can have an initial limit (per account) set by blizzard to have the launch balanced...
each character is identified by an ID number...prevent multiple copy of the same character from vanilla to tbc, unless the tbc char is deleted, in which case you could copy again...comparing the ID number at each copy request...
there you have it