Is it permissible to convert the mobile mosque caravan—for which a permanent mosque has been built in its place—into a guard's office?

The caravan designated for prayer in the company does not fall under the rulings of a mosque as long as it remains the property of the company and has not reached the stage of a legally recognized endowment (waqf). Therefore, it is permissible to convert it into a security office.

However, if it has been endowed for prayer, it then becomes an endowment for Allah Almighty, ceasing to be the property of the company. It is then impermissible to revoke the endowment or convert it to another purpose, unless there is a benefit that accrues back to the endowment itself.