The phrase "retroactive change" means making a modification or adjustment to something that happened in the past, so its effects apply to events that occurred before the change was made. Full definition
Facebook rarely makes material retroactive changes to the audiences of content, but if it does in the future it will need to determine the most effective way to push users to opt in. (techcrunch.com)
Retroactive change from MFS to MFJ is authorized only by subsection (b). (money.stackexchange.com)
The article below is accurate for your 2017 taxes, the one that you file this year by the April 2018 deadline, including a few retroactive changes due to the passing of tax reform. (turbotax.intuit.com)