While I agree that the only constant is change I also believe that indigenous religions need to be preserved. It should be an individuals choice whether they convert to something or not. Throughout most of history people were not given a choice, it was convert or die a horrible death. Even now there is some missionary work that basically barters clean water and schools (that dont teach indigenous religions) for converting to their faith.
You made a good point about unnecessary suffering and I had not considered that aspect but also consider that there was a study on peoples happiness around the world and people who were seen as poor, many of them indigenous, have the highest levels of happiness and contentment while those same qualities are rapidly declining in our "modern" societies.
As for preserving them... recording them is one thing but reading about something and doing it are two radically different things. Religion is something that runs deeper and more intangible than any other experience or concept that I know. I don't know that its possible to record a feeling like that. I suppose voluntary isolation would be the best solution allowing them the freedom to live their lives the way they want to and if their religion changes it will be because they want it to not because someone sold them a new one.
I think the best way would be to say "Are you happy?" and if they say yes, leave them alone, they've got it right. I think all too often people say "Do you want this shiny new thing you didnt know existed, might make your life a little easier, and might make you happier (but won't)?"
One last thing... perhaps spreading awareness about the religions, in particular their unique qualities. A marketing campaign for each one, to educate people and help them understand and in turn respect their uniqueness. Who knows, some business man might realize hes incredibly unhappy and wants to convert and worship the beautiful fertility goddess that lives in ancient rain forest canopies (but only on Mondays following the full moon)!
-Peter

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You make a great point, people so easily accept their own religious idiosyncrasies but are so harsh when judging others. I think a lot of it has to do with what you were taught as a child, your core beliefs. I think that's why historically so many children were separated from parents and sent to religious or boarding school to "acclimate" them to the dominant religion and cultural way of life. To make it more physical, in the past this was done in more violent ways when armies conquered new lands as well.
I also think a persons religion CANT be "not true" as long as THEY believe it and they have faith in it, that makes it a belief system to me something that transcends facts. To me religion (faith) and science (facts) are like two circles that exist separately but also coexist and overlap. I wouldn't mix dangerous chemicals guided by religion but I also wouldn't ask a chemist to marry me to my wife.
I have personally experienced things that I would say are impossible if I had not been there so to me its not my place to tell someone else what is and isn't possible. We all perceive and interpret the same thing in different ways so our life experiences are all unique. Generally I think religious stories are metaphors born from a seed of truth.
-Peter Ochabski