Yes it can be tightened but make sure that both the dentists are in touch regarding you case to judge wether  the treatment is going right or not. Also when you move to US carry all the paper work with you showing the history of your treatment. Since braces tightening is not so tedious procedure so you have the option to either get it through a private orthodontist who will charge you around $300 max or can go to a dental school...for more info on braces visit the link...http://www.identalhub.com/articlecategory-braces-21.aspx