Spiritual Retreat: Changing from One Iʿtikaf to Another
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1767
It is not permitted to change from one iʿtikāf to another, whether both iʿtikāfs happen to be obligatory, like when a person has made one of them obligatory on account of a vow and the other on account of an oath; or, both are recommended; or, one is obligatory and the other recommended; or, one is to be performed for himself and the other on behalf of someone else (niyābah), or he is being hired to perform it for someone else; or, both are to be performed on behalf of someone else.
-Ayatullah Sistani, Practical Laws of Islam, Spiritual Retreat (Iʿtikāf)