Converting virtual products in Magento 2

You can do it only direct in DB. For example with query

update catalog_product_entity set type_id = 'simple' where type_id = 'virtual';

But beware, this query converts all virtual products to simple. (better to create db backup before start)

Maybe this helps somebody who looks it up.

If you create product configurations and dont add weight to the generated products they will be virtual. If you add weight they will be simple products.

If you want to change a single virtual product to a simple product, give the product a weight in the backend and change it to "this product has weight".

However if you want to change all virtual products at once you could add a weight to all selected products at once by using the "update attributes" option under "products->catalog". Meanwhile you cant set the "this product has weight" option. So you probably have to do that in the DB or per product.