Question about IMMUTABLE function

Hi
Documentation states, that IMMUTABLE "... guaranteed to return the same 
results given the same arguments forever." My doubts are regarding 
'forever' word.
Q1: Can I safely mark as IMMUTABLE
     FUNCTION calc_col_gen_full_weburl(content_pages_table)
Which is used in following context:
     SELECT p.id, p.calc_col_gen_full_weburl FROM content_pages_table;
Given that:
     1. It internally does SELECT. It looks at other rows in the same 
table (parent rows, to generate full url). And these rows may be updated 
some day. So it should return new value for row#5 if row#3 will be changed.
     2. It in turn calls other function that is really IMMUTABLE (say 
my_char_replace()) and does not selects from anywhere. But, the 
implementation of that (my_char_replace()) function may be changed 
someday (it will be recreated).