No, that would not make it a balanced first stage, Not even close.
The inside barrel on a Conshelf has a lip specifically designed to support and lock the balancing chamber. Without that lip the pressure thrust on the back of the balancing chamber will just push on the seat. Totally disabling the balancing chamber function.
The volcano orifice of a non-balanced regulator is also a different diameter than in a balanced diaphragm regulator. There is a reason for that. The explanation is not complicated, but I would recommend reading Regulator Savvy Book, by Pete Wolfinger.
https://www.scubatools.com/c-23-regulat ... -book.aspx
https://www.scubatools.com/p-603-regula ... bound.aspx
Regulators look simple and are very simple, but there is a bit design that goes on the sizing of the components.
I would not be mixing components from totally different regulator designs unless they are the same dimensions or you understand the pneumatic function and the consequences of the changes. Keep in mind that just because something looks like it works, it may not work in all conditions.
Does the seat really need to be replace? You may be able to polish the surface.