Менять NEW смысл имеет только в BEFORE-триггере.
32.1. Overview of Trigger Behavior
The return value is ignored for row-level triggers fired after an operation, and so they may as well return NULL.

35.10. Trigger Procedures
The return value of a BEFORE or AFTER statement-level trigger or an AFTER row-level trigger is always ignored; it may as well be null. However, any of these types of triggers can still abort the entire operation by raising an error.

Нужно писать обычный UPDATE (и при этом заботится чтобы не возникло бесконечного цикла).