Re: You are referring to trans-oceanic flights?
Who gives a rat's ass about trans-oceanic flights. Even that will be possible I am certain. Hell, it's possible for a human-powered aircraft to cross the English Channel isn't it? It all depends on the aircraft design. Very likely a simple conversion to hydrogen won't do. The aircraft will have to be completely re-designed from the ground up. Conversions might be ok for short- & medium-range flights however.