If travel was generally budgeted at time of proposal/award, then I would suggest checking the budget justification, NOA, and NOGA to see how travel was described in those documents. My understanding is that the detail surrounding whether travel is domestic or international is not always clearly delineated based on the sponsor. I think NIH is one of those sponsors. NSF seems to make applicants clearly state whether travel allocations are domestic or international.
If the grant has broad rebudget authority (scope unaltered), then you could potentially add the needed travel account code if it was never budgeted/requested. However, I would stress that a very strong justification about how the travel benefits the grant research aims is needed. I would also recommend getting a clear statement from the PI that this change will not alter the work scope.
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Given that the Fly America Act language should be on all NOGAs for federally contracted funding, I would not assume that international travel is allowed just because that statement is included.