PL/SQL: numeric or value error – ORA-06502

You tried to assign a value to a varchar variable, but the value is larger than the variable can handle.

You are going to select

TO_CHAR(t_old.var_v_2) AS two - number(8)

into

var_v_2 - varchar2(4 char), 

You can’t select 8 characters into a variable, size of 4 characters.