The ability to bulk reformat text is a basic Scrivener feature. Refer also to §15.7.5, Resetting Formatting, in the user manual PDF. Comments are reset as described in §18.4.2, Resetting Linked Note Formatting.
As for fonts though, I’m not sure if you can (licensing issues and such), but I have found that if you install your favourites into Linux, Scrivener will pick them up and use them reliably. It’s more an issue going to or from Mac or iOS, where you may see different platforms getting confused over how fonts are described. In this case particular case though, it is the exact same software.
So you might want to look into that, and seeing if the sections you haven’t touched yet load in the preferred font automatically, before changing everything (not that it is particularly difficult to change things back, if you later install fonts).