Update all the posts without delete it all

Hi,

I host a sites with more than 40 000 posts. And could happen that we need to updates all of them. Since this process will take almost 2 hours, I would like to know if there is a way to index the document only for updating and not for deleting and index from 0. This will allow me to continue to have the search always online.

I would also know if this is not possible in the admin panel, if we can do that by cli o calling some php functions.