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I can't imagine why there would be any limit. Though these things are much more valuable than the meager mileage that they give you.
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Exchanging them for miles towards 1/2MM is my goal. Since most of my AA travel will be intl, I dont see any use for the upgrades.
I primarily use HP for domestic flights. They have upgraded me for free on all but two of my 37 flights so far this year.
If all goes to plan I should collect around 40+ upgrades throughout the year. That comes out to 100k or 10% of the 1MM requirement.
That to is more valuable to me than the domestic upgrades.