It is sometimes possible to flatten a formed piece to once again fire it in a mould. A lot depends on the shape of the mould it was first fired in. A mould which requires a lot of stretching or has sharp angles is likely to produce a flattened piece of glass that will have extensive distortion of the pattern or design of the original piece.
A full fuse is not necessary. The object is to make the shaped piece flat. That should happen at slumping temperatures and certainly long before a full fuse. A tack fuse temperature may even be more than required.
You will need to observe the firing, to ensure that you get the piece flat but do not further distort it. Set the schedule for the temperature you think will do the job. Then add some soak time. Observe with quick peeks from about 50C below your target temperature. This will give you the information you need for this one and future pieces.
When the flattened shape is achieved, advance the controller to the next segment and anneal as before. When cool, wash and inspect the piece for a clean surface without any imbedded kiln wash or fibre paper. Consider whether this flat piece still has a desirable design, or is too badly distorted. It may be that you want to slump the piece even if the design is distorted as a learning experience.