It's always tough to pinpoint the exact moment something changed... if you'd like an easy answer that satisfies most people, Deedrah's Reload (3D Vision Records, 2000) is the first unambiguously full-on psytrance tune with that distinctive rolling bass line. Several other 3D Vision affiliated artists had already blazed the trail; Talamasca's debut album Beyond The Mask certainly has full-on characteristics; Absolum's Regenerate was a super strong tune also released in 2000; and then you've got oddballs like DJ Maël & Manitù's Zergshake.
Some others worth considering: Mumbo Jumbo - Weird, Sick, and Twisted (2000); Space Cat's entire Shapes of Sound album (2000); Tristan - Small Paper Squares (2000); Xerox & Freeman & Liquid Metal - In My Brain (2000); and maybe, if you stretch your ears a little, Children of the Doc's Police 106 (1999), fittingly released on one of those old Full-On compilations from Hommega.
It is interesting how quickly everything changed too... step forward one year into 2001 and there is no shortage of examples to consider. This is when GMS steps onto the full-on scene (I'd argue all of their pre-2001 releases are still classic gritty GMS), Astrix, Alien Project, Bamboo Forest, Bio-Tonic, Earthling, Logic Bomb (rapidly maturing after the experiments of Headware), Neuromotor, Silicon Sound, Space Buddha, etc., plus many more developments from Deedrah, Talamasca, Nomad (Mael), and the rest of the 3D Vision gang, who I would personally credit for the development of full-on psytrance.