Change Car Class / Tune Pre-Race?

Where in the game are you able to do an S1 race back-to-back with an S2 race? Is this online? Which mode?

Yes, online. Mode? Online free roam for example! Who ever starts the race picks the class. Now if you have an A Class car, say a ZR1 Corvette but have an S1 build for it also that you have saved in your tunes. If the race comes up S1 then you can't see
or pick the car because it's currently tuned A. So you have to have two or three of every car and each one tuned to a different class to cover the game options.

Interesting - I haven't done much online in this game so I haven't run into that.

That does seem disjointed from other Forza titles which would allow you to choose from any car that "could" be eligible for the chosen class (and then you'd apply a tune or auto-upgrade to get it in the right class for the race).

Probably best to get some cars in your garage first, you get a car for free every time you level up (up to level 50) You'll also definitely need to tune the car if you want to be competative on-line. If you don't know how to tune then use the storefront
and buy set ups off of that. Be careful though as there are a lot of garbage tunes out there.

When racing on line you can choose any car from within your own garage as long as it meets the classification for said race e.g A class, B class etc

I see. So basically, you like to do what I call street tuning. That's not to say you don't use any race upgrades, just not the aero packages. I have a few cars I've done the same with myself. I'll use the aero packages on fully blown race tunes but that's
only for A-class cars and up. Mostly anything below that in my garage is tuned without the added downforce from an aerodynamic set.