Your web storage won't be affected negatively. Rather the contrary! And search engine rating for your forum will increase too when using the more contemporary media file formats.
I see now reason why the forum enforces me to use PNG for lossless screenshots although WEBP is the more modern format, often with a better lossless compression. Hey, you can even make WEBPs in GraphicConverter
Also why not a smaller more efficient MP4 rather enforcing its 30 year old counterpart aka Animated GIF for screen recordings or mockups?
I hope this changes.