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The $742 fare looks pretty reasonable. Considering that a change fee would be at least $150 plus fare differences getting the $570 ticket now and hoping to change it later doesn't make a ton of sense. I suppose there is a chance that the longer stay ticket will drop down a bit but it is hard to know one way or the other for certain.

I'm also not sure what the visa rules are and if you're allowed a 5 month stay. If not that ticket wouldn't work so well for you.