Usually, during "The Call" the agent will ask if you have other work. (Or, if they don't, you can mention it).
My agent knew I had two books completed in my adult scifi trilogy when she signed me, based on CROWN OF ICE. She wasn't that interested in the adult scifi at first -- I actually wrote another YA (a scifi) and she sold that (and its sequel) before I mentioned the adult scifi again. Of course, in the meantime I'd gone back to the adult scifi and revised it (a couple of times).
She is now going to start submitting the adult scifi in Feb. So yes, you can go back to older work and the agent may be interested. (May be). It doesn't hurt to mention it when you talk to an agent, but just don't make it a deal-breaker ("Take all this on or I'll say no" -- LOL).
You do pitch new projects, but it's a little different -- it's more about pitching different ideas and seeing what the agent thinks is best to work on at the moment (at least in my case). You aren't "querying" all over again.