Ultimately, one must protect and ensure the perpetuity of a waqf asset. If there is a need to trade the shares in order to acquire other shares in a different company then it is permissible.
The main principle in this issue is, can we exchange a waqf asset with another? If the endower gave the permission to do so at the time of making the waqf, then all scholars agree that there is nothing wrong in exchanging it for another asset.
If the endower did not give his permission, then the scholars differ on this matter with some believing it is permissible to exchange the waqf asset, when it has cessed to produce any benefits.
If the endower strictly forbade the exchange, then the Hanafis, Malikis and Shafis it will not be permissible to exchange the waqf asset with another under any circumstances. However, according to the Hanbalis, if the waqf asset does not produce any benefits anymore, then it will be permissible to exchange it.
And Allah knows best!
(Mufti) Billal Omarjee