My project  has a lot of GIF images on all pages as it's a library. This makes the page size huge. So I decided to turn all animated GIF images to PNG image sprites (which reduces image size dramatically) and animate them by JS. But now question appears - the project has been showing those GIF images for years, how will that impact SEO? Will it create problems with search engines?
SEO impact: changing animated GIF images to PNG image sprites
seo;google search
Have you tried optimizing the GIF images? Properly made GIFs usually shouldn't be much larger than PNG sprite.But as far as SEO goes - it shouldn't have much, if any, effect on rankings. Just make sure your JS solution works on all browsers and devices. And maybe have the old GIFs as fallback in noscript tags.