I adamntly disagree with Roger but first let me answer your question. Once you have changed the ASA you will get a new combination of shutter speed and aperture for a Zone I exposure, use the new setting. Practically what you are doing is taking diferent exposures by 1/3 around the manufacturers recomended film speed. For example if your inital exposure is for ASA 400 and then you reset the ASA to 320 then what you actually did is overexpose by 1/3 stop.(posted 8300 days ago)As to Roger's comment I could not disagree more, sensitometry is only a TOOL of the zone system, and not a dificult tool to use at that. It might have confused him, but do not let him discourage you in using a tool that will allow you to diagnose a problem in the future if your film is not comming out as you want it, it will allow you to compare different films and different kinds of proccesses with the same film, etc. Beleive me the time you are spending now learning sensitometry will save you a lot more time and effort in the future.
The zone system will allow you to be confident in your choices of tonality, it will allow you to determine your mistakes in exposure and it will allow you to determine the limits of your film/developing combination. In the end the zone system is also a TOOL and like every tool some people are better at using it than others.
The only one thing I will mention about the zone system is that like every new tool you tend to overuse it at first, and spend a lot of time doing tests and not taking pictures, don't fall into this trap, I did and realized that I was testing for every film under the sun in search of the "perfect" film/developer combination. There is not such thing and here is the beauty of the zone system, it will tell you what the limits are and what films would be better used under different circumstances. WHy do you think that many LF photgraphers use more than one film? It has to do with aesthetic choice and knowledge of the film, some gained through empirical experience and a lot of money wasted, I imagine this is the case with Roger, and in most cases with experienced gained by doing tests and understanding how a film behaves.
You have the AA book, in my opinion it is the best, it is simple, it tells you what you need to do and it moves on into the real meat of photography which is making informed choices about the tonality of an image you wish to record. If you use Roger's recommendation and develop a little more here expose a little less there what you will end up is a method in which you will not be able to determine why things went wrong, and when they do work your print will probably look all the same, as I suspect his do. Good luck, and stick with it, trust me on this is time well spent.