If I change a PK of some Table and it is related 10 objects ...

Hi i`m junior developer in JDE

please advise me ^^;;

I will modify a PGM and..
First I will change a PK of extension TABLE and..
This table is related 10 objects are Batch PGM like R59K..., R59....

I heard that If I change a PK, I must modify related object. is it right? from another JDE developer.
I don`t understand it.

I just change a PK like that
col1+col2 --> col1+col2+col3

If is it true.
Why? and How I modify related PGMs?

please advise me please.

thanks