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In theory yes they could be changed without removing the heads, the same way you would do them on say a small block chevy with compressed air to keep the valve from dropping into the chamber.
Unfortunately because of the tool needed to install the seals you would not have the clearance with the frame to do this, so the answer is no, you must remove the heads. Also at this the you would want to check the valve guide clearance and see if they are good also.