Does performing ghusl after assuming ihram and before circumambulating the Kaaba in Mecca affect the validity of Umrah?

There is no harm for the Muhrim (pilgrim in a state of Ihram) to bathe whenever he wishes, as this does not violate the state of Ihram. The jurists have stipulated the desirability of bathing upon entering Mecca at Dhu Tuwa. Among the recommended baths for Hajj are: bathing for Ihram, for entering Mecca, for standing at Arafat and Al-Mash'ar Al-Haram, and three baths for stoning the Jamarat during the days of Tashreeq.