Oh, it’s a good one... I had always assumed that the “you can’t activate opportunities of the original element” only applied to the generic Invocations opps on page 191 - and that you could activate the opportunities of the substitute element from that page instead - but that the opps linked to the spell itself could still be used. Now I realize I may have been wrong.

Narratively, my take is that you really only use the new element to achieve the same mechanical effects. So in your example, no flames at all! The Shugenja just summons a gush of Water that deals damage as per Fire from Within, but equal to their water ring instead of fire ring. If you want it can be boiling water, but it’s still water

"When performing an invocation, you may choose a different ring with which to make the check. While using this school ability, you cannot activate * of the original Element of the invocation, but you can activate * for the new Element. You can use this school ability a number of times per game session up to your school rank."

"Fire *: The weapon you summon can be any kind of melee weapon instead of a katana."

Yeah....it specifically namechecks the element in invocation opportunities, so no, I would say you can't trigger opportunities linked, but will only get the 'core effect' of the invocation. Plus, if you're using it with a 'better' ring, then you should be racking up more bonus successes and opportunities, even if you have less options to spend them on.

You do get a new 'arsenal' of abilities; being able to use "*+: During an Attack action check, ignore 1 point of target’s physical resistance per * spent this way." on what should be an air invocation is pretty nice (fire 'attack' invocations tend to be supernatural damage anyway, so already ignore armour, but the general point still stands)