Praise be to Allah.
The one who does ‘umrah has to shave his head or cut some of his hair, but shaving is better. He has to remove hair from all of his head and it is not sufficient to remove hair only from part of the sides of the head, according to the more correct opinion. This is the view of Maalik and Ahmad. Abu Haneefah (may Allaah have mercy on him) was of the view that it is sufficient to remove hair from only one quarter of the head.
Al-Shaafa’i was of the view that the minimum that is sufficient is three hairs, to be shaved or cut.
But all are agreed that it is better to shave the whole head rather than cutting the hair, because Allaah says (interpretation of the meaning): “having their (head) hair shaved and cut short” [al-Fath 48:27], and the word head means the whole thing. And it is proven that the Messenger of Allaah (peace and blessings of Allaah be upon him) shaved all of his head. See: al-Mawsoo’ah al-Fiqhiyyah (18/98).
The argument of the Maalikis and Hanbalis is that the Prophet (peace and blessings of Allaah be upon him) shaved all of his head, which explains what is meant by shaving.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (3/196): He has to cut or shave all of his hair, and the same (for cutting the hair) applies to women. This was stated by Imam Ahmad, and it was also the view of Maalik. It was narrated from Ahmad that it is sufficient to (cut or shave) some of it. al-Shaafa’i said: It is sufficient for him to cut three hairs. Ibn al-Mundhir was of the view that it sufficient for him to do what may be called cutting short. The fact that it is obligatory to include all of the head is indicated by the words of Allaah (interpretation of the meaning): “having their (head) hair shaved and cut short” [al-Fath 48:27], which is general in meaning and implies all of it. And the Prophet (peace and blessings of Allaah be upon him) shaved all of his head which explains what is meant by shaving. End quote.
It says in al-Taaj wa’l-Ikleel (a Maaliki book) (4/181): Whoever shaves his head or cuts his hair, let him (shave or cut hair from) all of his head; it is not sufficient for him to remove hair from only part of it. End quote.
But if a person removed hair from only part of his head, following the view of those who hold this opinion, then he does not have to so anything now, because it is a matter that is subject to ijtihaad and the scholars differed concerning it. Based on that, you do not have to enter ihraam again and shave it or cut it again.
But if you do ‘Umrah or Hajj in the future, your cutting the hair or shaving should include all of the head.
And Allaah knows best.