AT&T Viva Mexico Question's?

Yes whenever you change rate plans there can be some pro-rating depending on the day and where you are in your billing cycle.  But it equals out.  You can minimize it by changing on the last/first day of your billing cycle and customer service can help set that up.

No there are no roaming fees.

http://www.wireless.att.com/learn/international/viva-mexico.jsp

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Get the benefits of an AT&T NationSM Plan, plus no more long distance or roaming charges for voice calls between the U.S. and Mexico.

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Yes, rollover minutes is a plan feature.

http://www.wireless.att.com/learn/international/viva-mexico.jsp

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Rollover® Minutes, only from AT&T

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Now if you change to the 1400 plan you will lose any rollover minutes you have that are in excess of 1400 when you make the plan change, and then you start accumulating them normally after that. 

http://www.wireless.att.com/answer-center/main.jsp?t=solutionTab&ft=searchTab&ps=solutionPanels&locale=en_US&_dyncharset=UTF-8&solutionId=52371&isSrch=Yes

do i stay need to stay full moth in usa for don't get extra charge in plan or because I travel a lot to mexico something I need to stay weeks