An interesting Q&A on the ISAF website: At an obstruction the boat with right of way has the right to choose on which side to pass the obstruction. Written in the new rulebook in rule 19.2(a). Pretty straight forward, don't you think?
This Q&A answers a question when the right of way switches just before the obstruction or is subject to change because of boats changing course.
I think this Q&A perfectly illustrates the "new" principle that the rules in sections A & B are no longer "switched off" for passing marks and obstruction.
Like on the beat or on the run or anywhere else on the course, the first thing you need to establish is, WHO has right of way.... and only then can you use additional rights and obligations from other rules.
Read the Q&A here: Racing Rules Q&A 2009-021