That long string might stretch 3/4" - 1". Most of the stretch will occur in the first 100-200 shots, increasing your draw length and possibly affecting the performance of the cam. All you need to do is put some twists in the string to shorten the string and get the draw length back to where it should be.
If you want to spend the extra $$, some string materials stretch less than others.
I agree that if the string looks OK and the draw length is OK, you can take a chance and leave the old string as is. If the string has stretched but is still in good shape, just twist it up as mentioned. Or, you can just go ahead and put a new string on and remove all doubts about the string's condition.