What Nick said re' the filtering... In practice, you will find you need to adjust exposure somewhat in any case because the different filter grades effect highlights and shadows differently, so the balance of light/dark is altered somewhat between grades.

I have not made the switch from condensor to diffused, but my understanding is as you say: increase development slightly. From my limited experience (and a lot of reading...) I do not expect you will need to alter film speed as the increase in development will probably not be sufficient to change the toe of the film curve by much. However, if you find you are now printing on to grade 2 or 3 for most of your negs then you may as well stick with your development times as they are. But, I'd wait for more experienced people to chime in before I took my word for it...

Cheers, Bob.