If you mean a DSL modem or cable DOCSIS, I don't think those are easily replaceable. But you can definitely put an OpenWRT device right behind it and use that. It's pretty straightforward (plug in the upstream side, wait for it to get an address, done).

As for how much you need to know... okay. That's a tricky question because, the most you mess with OpenWRT, the more some stuff becomes automatic, and that makes it easy to forget things. That's not on you, that's on me.

That said, thinking about it a little, the defaults are pretty workable right after installation. You'll have to set an admin password on the OpenWRT box (it nags you until you do these days), which should be familiar. Turning up wifi is a little tricky at first. I would recommend reading through the quickstart guide once or twice before digging into OpenWRT configuration because it lays out all of the basics that you need to get going. It's about as well written and useful as the manuals for access points were way back when.

One thing I would recommend is, if you build an OpenWRT box, setting it up before you plug it in and use it as your network gateway. It's much easier to poke at it without having "When is my network going to come back up?" rattling around in the back of your mind.