Don't modify the behaviour of your BLL and DAL... just use the built in tools to extract interfaces for them: Extracting an Interface - MSDN (by "directly modify", I assume you mean completely refactor).
Then you will have interfaces with which you can start re-implementing as you fix up the BLL and DAL later on down the track.
There is no other way around the tight coupling here. If you must instantiate an object directly.. you've automagically coupled them. At least once you extract interfaces, your dependencies become inverted (see: Dependency Inversion Princple) and they can be injected into your services.