'value too long' and before insert/update trigger

Hi all
I'm currently migrating a legacy Informix 4gl application to run on PostgreSQL (v9.5.8)

There are errors occurring because sometimes the application tries to insert/update values longer than the definition of the database column.
The error message is eg. "value too long for type character(20)".

The behaviour under Informix is to silently truncate the saved value, but with PostgreSQL it crashes our application program. Obviously the short answer is to modify the application so that it does not attempt to save values that are too long, but with 1000+ programs and approx. 8000 insert/update statements to review this will take a lot of time.

I was hoping to emulate the Informix behaviour by creating triggers to run before each insert/update that would check the length of the values being saved and truncate if too long. However I'm still getting the error messages. Presumably the length validation is being done before the trigger is run. Is there some way this could be changed so the trigger happens first?

I've seen examples that involve changing the char type columns to text, but this changes the semantics of string comparisons with regard to trailing spaces, and I'm concerned that this might cause subtle and hard to find problems within the application.

Thanks
Kevin