Underslab drain replacement not deep enough for current code

Remodeling a house in Colorado in expansive soil. Had to remove the slab on grade due to heaving. Will replace with a waffle mat slab. Seems like a great, proven solution to the issue. The foundation is caisson and is in great condition, the issue was the floating slab poured directly on the ground.
Found the lower half of the cast iron badly deteriorated by the damp soil under the slab and it has been removed.
The current pipe is 3" iron.
Permit is pulled and city follows IPC. It appears that the upper end of the pipe is not deep enough under the slab (12" required) to meet updated code. City said they will not require that the existing pipe be brought up to code, but what if it should be replaced with PVC Schedule 40? Upper end was higher than most underlying soil and in dryer soil and did not corrode--looks to be in good shape.
What would you do? Just replace the rotted pipe at the lower end (this part is all 12" under slab) with 3" PVC? This will meet code and should be only the pipe changes that will be inspected I think. Or replace it all with 4" PVC? (appears to only be 3" running to the septic system). Or replace it all with 3" PVC?
Remember soil is expansive so plastic might be susceptible to cracking. Structural engineer recommended Schedule 40. Only have one chance to fix this, will not be removing the slab again.

My question must have been too involved. I am going to replace all the pipe now.
I spoke with the plumbing inspector and he gave me the go-ahead to replace all the pipe at the current depth. He said even if the code now says 12" below slab, it is not something they require when replacing existing pipe. He recommended 3" schedule 40 to the 3rd toilet and 4" from there to the building exit. I read that only 3" is required for 3 toilets, but I guess that gives us some leeway for the future.

In Washington State, I've used 3" pipe for three toilets, and bump to 4" where the forth is added building sewers have always been 4" outside the foundation regardless.
But when in doubt, or if you have a handy inspector, it's best to just follow his lead.

As far as foundations go, the normal requirement on footings is that they be below the frost line.

Thanks for your answer. The 12" below slab is one of those things I could swear that I read in the code, but can't find again. I searched for the statement that I thought I read and can only find some rules about trenching and filling in with clean dirt. Asbestos pipe requires a 12" deep trench, but of course I'm not using that.
I have increased the size of the drain pipe at the third toilet to 4" and will connect to the outside 4" drain through the existing hole in the foundation.