...Can we expect all Kodak films will be made on PET in the near future?...
No. Kodak films will continue to be made on the same base materials they were previously. Those films on acetate bases will, after Kodak's existing stock of in-house manufactured acetate base runs out, be coated on acetate base purchased from outside supplier(s).
...Does it mean the (Tmax400) 120 is made (today) on a PET base, and if so, is it prone to light piping?
No. 120 400TMX as well as 120 100TMX are and will continue to be coated on a 4.7 mil acetate base. Note that this is considerably thicker than other 120 roll films, except Delta 3200, which is cut from the same parent rolls as HARMAN's 35mm version of that emulsion.
The base thickness of 120 TMAX films is why there were never -- even when 220 was popular -- 220 versions of those products. Twice the length wouldn't have fit on a spool. An interesting historical footnote: when first released, the 120 TMAX films were on an even thicker base, right at or just above 5 mil if memory serves. This caused transport problems, especially with Mamiya cameras, so Kodak "slimmed" it down to 4.7 mil.