Thank you for pointing me to the right direction. I noticed that this does not animate, but does change the aspectRatio:
gsap.set(svg,{attr:{preserveAspectRatio:"xMaxYMax slice"}});
gsap.to(svg, 1, {attr:{preserveAspectRatio:"xMinYMax slice"}});
So by first animating the viewbox, and the end of the animation changing the preserveAspectRatio that way, I could establish what I wanted. Thanks!