I see no reason why you wouldn't be able to replace the tweeter. Just keep in mind the crossover and baffle changes that JRH referred to.

However, I'm sure you already know but, you should realize that you may not like the results of the modification and you may be unable to reverse the changes you made (especially if you cut the baffle). Also, you may have a very hard time if you decide to sell the the speakers after the modification.