When the automobile was first introduced it was required that a person with a red flag walked in front to warn people of the danger it represented.
Initially there will be exemptions from the rules on a trial basis. but eventually the rules will be changed to accommodate changes, here as in all other cases of new inventions.
It is already becoming clear that to take an existing vessel with conventional machinery and equipment is NOT the way to go.
Hydrogen fuel cells are fast developing into viable power source for propulsion, even for large vessels. But initially the technology will be tested on small vessels in domestic and short sea trade.
A new company dedicated to developing solutions for maritime use of hydrogen as power source was officially launched yesterday: http://nelhydrogen.com/news/hyon-was-launched-today/
PS> If you have not already followed the conversation in another thread here on the forum, you will find a lot of informations and arguments, fore and against here: Future of ships