Gentlemen has 1,000-gallon drain field that is bad. Current it is
A 10 foot corrugated plastic pipe that goes out into the drain field
Maybe it is about 36 inches deep. The pipe has holes in the bottom.
With gravel underneath and tar paper over the top.

I see they have a new idea: This load bearing, long lasting French drain is easy to install and is light weight. The lightweight poly rock takes the place of gravel and the pre-assembled benefit of the EZ drain removes two steps from the traditional installation of a French drain. It can be used anywhere a gravel/slotted pipe combination is currently used.
I wonder if this would be an option saving on gravel and tar paper. Would it fit the outlet fitting from the drain field to the leach fields?

I am guessing they have a 1,000 gallon septic tank. Only 1 field pipe? Have them look at the "Infiltrator" system. Most places in the Atlanta area no longer allow the old perforated pipe in gravel.

As far as fitting new pipe into the existing outlet - I see it as no issue. Today, you can purchase fitting that will fit any pipe configuration and material, on each end of the fitting

Check Fernco catalog for more details:

https://www.fernco.com/

Milo