The US, along with Eritrea, is one of two nations to tax by citizenship, which means expats are still expected to file returns or risk penalties.
Earlier this week the Internal Revenue Service gave Americans with liabilities of $1m (£0.75m, €0.85m) or less more time to get their affairs in order.
Marylouise Serrato, executive director of ACA, said it was the best chance of pushing through the changes expats have campaigned for.
“We do feel this opens the door to further reform,” Serrato told International Adviser. “The delay signals to us they acknowledge there is a problem.
“There is a dialogue on this issue like there has never been on a move from citizenship to a residency-based taxation.
“We are knocking on doors and meeting with members of the Ways and Means Committee and the Senate Office Committee – it is a challenge.
“People agree it is the right thing to do, we just need that groundswell.”
The complexity of the transition “Apple” tax is one of ACA’s complaints.
It is aimed at encouraging corporates to bring money onshore into the US or face a 15.5% charge.
However, citizens with ‘mom and pop’ businesses abroad are also covered, as are freelancers paying themselves through companies.
The new deadline is 15 April 2019 for those covered by the reprieve, as opposed to the original deadline of 15 June 2018 which still applies to everyone with $1m+ liabilities.
A special, but narrowly worded, rule gives additional relief to some Americans abroad, “whose tax homes and abodes, in a real and substantial sense, are outside the United States”.
They have been given two additional months until 17 June 2019.