Same deal here. Replacing the pipe to the septic tank would not require a permit or inspection. It falls under the heading of repairs and therefore doesn't need a permit or inspection. Same for replacing a septic tank with a new one.
And you can do repairs to the drainfield itself (should you have a collapse cause a heavy piece of machinery or similar ran over it)... without a permit.....cause it's a repair.
However, if your drainfield becomes totally exhausted and you need to place a new one.......that requires a permit, inspection and the whole nine yards.
But you need to/should check with *your* county or city/township health department because all these things vary by jurisdiction. Should be able to find out with nothing more than a phone call.