converting crawl space to concrete slab. can I leave existing plumbing?

I have a 1920 home over a crawl space with pier and beam foundation.   it has a double block footer all the way around.   the wood is all rotted and crumbling so I have been told converting to a slab is the right way to go.  basically fill in with gravel and sand. then pour a 4 inch slab to be the new subfloor for the house. my question is regarding the existing plumbing.  can I leave it in place? or does it have to be removed before the fill is put in. then rebuilt before the slab?

thank!

I wouldn't. Drain lines may be okay, supply lines are a big risk in my experience. If your pipes spring a leak it takes a very long time to notice in a slab. By the time you notice a LOT of damage could occur as your walls soak up the moisture from the concrete. Then the repairs would include having to trench through your slab to access the pipes which is a costly and MESSY process.

A lot of old houses have electrical wiring under them as well, make sure that isn't your case.

If questions or doubts, call up a local plumber. Ask them their experience with slab versus crawlspace. Most experts will willingly share their stories. May even be worth a consultation fee.

What climate do you live in?

I'm having trouble figuring out the current geometry.  Is the double block footer two cement blocks high or two blocks wide?  Are the piers sitting on that footer with the beams resting on them?  Are the piers the only rotting parts?  What makes up the walls of the crawlspace?

Not knowing enough about your situation, my default would be to keep a crawlspace if it's reasonably possible.

Depending on the design of the house, replacing the timber may be better.
Restumping etc is not difficult if your floor is out.
And you can do it slowly, one section at a time.
I could teach you over this site.