"As recently as Oct. 19, the NIH defined “gain-of-function” experiments as “a type of research that modifies a biological agent so that it confers new or enhanced activity to that agent.” The NIH noted that “This research poses biosafety and biosecurity risks,” and warned that “these risks must be carefully managed. ”

Source

Why did the NIH change it's definition on Oct. 19th?

The "gain-of-function" definition is just deleted.
https://www.nih.gov/news-events/research-involving-potential-pandemic-pathogens

I'm sure a quick Google search will give you a "debunking" story you can parrot.