Question:
Zaid is addicted to pornography. So he makes taubah and makes a نذر that he will keep 10 days fast if he was to return to it. But later his nafs get the better of him and he returns to the sin. Knowing that he won’t be able to fulfil his نذر he changes it to not eating lunch the next day. Then to a specific istighfar(سيد الاستغفار) then to one month of continuous fast.
My question is:
1) Is it permissible to change the نذر?
2) Does he have to read the specific istighfar?
3) How grave is the sin of pornography?
4) After changing his نذر does he still have to fulfil the other older ones that he made ?
Note
He changed all of them after engaging in the sin at least once.