Simply create a normal (non-virtual) column with an Initial value expression the same as the App formula expression currently used by the _ComputedKey virtual column. Mark this new column as key and delete the _ComputedKey column.
If your app currently references the _ComputedKey column explicitly (in expressions, e.g.), you could name the new normal column _ComputedKey. When you regenerate the column structure, the virtual column will be replaced by the normal one, and (if I recall correctly) the rest of the old virtual column’s configuration will be applied to the new normal column, making the transition a little easier.
Note carefully: No matter how you implement your key column, either by virtual column or by normal column, the key value cannot change after its row has been saved the first time. A computed key will only ever be computed and changeable before that first save. After that, it’s fixed in stone.