Judasmac, what is the problem with the woofer? It may be able to be repaired.
Yes, swapping a driver with one with much different characteristics will not be uncomplicated. Before you decide to undertake this proposition, I would look into the crossover. You might want to throw it at us. Generally, you are going to have to relieve the padding on the tweeter, ala lower/fewer resistors.
While many will start to curdle with fear, if you are intent on keeping these speakers, and the woofers cannot be repaired, you may have to explore this.
As a suggestion, I would contact with REAL WORLD experience in loudspeaker repair. He will be able to guide you. If you cannot locate such a person, let me know, a dear friend of mine is quite probably the best in the USA - I can put you in contact with him.