Exactly. I like the way you think @Handy1 . The only the only thing is to change the plan before they go and to start on the next billing cycle. However if they start it earlier they'll lose what they've paid for.
With that being said, @antoniosales when is your billing cycle and when are you heading to the US and for how long.
The 24 hour wait is only if OP wanted to re buy the very same US roaming add on . Otherwise they can purchase a different US roaming add on . But at that point it would make more sense to go with a CAN /US plan cheaper and more data much more
