How do I joint the subdivisions surface head back to non-subdivision surface body without seam?

I separate head to apply subdivision surface modify and I want to join it back to the body that still have shape key but there is a seam between head and body after joined. How do I fix it?

Normally if separate the head or something and join it back to body after use merge by distance it will get back to normal mesh without seam but in this case I can’t apply subdivision to body because I want to keep the shape key.