I have an old tile shower that was leaking and was going to put a new enclosed styrene unit in instead.

I'm kind of scared to start jackhammering to remove the old mortar base - won't I crack the drainpipe? Would it be OK (or wiser) to leave the old floor in place, build a platform above it, extend the drainpipe upwards, and put the new syrene floor on that?

Parry

Is your drainpipe PVC? How old is the existing structure?

When remodeling, and you demo a structure, you basically should take time to replace those items that are old and need replacement. Do you know the condition underneath the shower pan?

Take this time, and some extra money, to replace those things that you might be covering up by taking a shortcut and becoming more costly in the long run.

Besides, I don't think your plumbing inspection would pass with what you are proposing.

DaBa is correct, you should not try the short cut you describe. It will not be fun riping out the old shower pan but it will come out. You say the old pan is leaking, do you know where the leak happens, is it just bad grouting? Unless you are itching for a remodel some folks here may be able to help you repair your leak.