Hi,
You could alter the part number in the "properties" dialogue box when the flat pattern is activated.
The fundamental problem here is that Autodesk has assumed/determined that 2 things (flat pattern and pressed part) are the same thing, when physically in a wide range of properties they are not. You cannot have a components that are different shapes (compare the bounding box dimensions of a pressed part and flat profile they are always going to be different) to one another classified under the same part number.
Different shapes is the major determinant as to why the part number should be different between flat pattern and pressed part but also imagine a handed/ mirrored pressing. The flat pattern may be the same, but from that flat pattern you can press two different types of pressed component. It's worth noting that we don't but it does sort of highlight another reason to have the flat pattern as a different part number.
Finally it has massive implications for historical data that is going to get wiped out. We have circa 5000 flat pattern part numbers that will be wiped out with no option on how to recover them or more importantly relate them to the flat patterns in both the modelling environment and the drawing environment. This is a massive ball ache and to have the option of how we have been doing it taken away seems unfair.
P.s a really good way to highlight this is to produce a drawing of a pressed part, add a part list to the drawing sheet this will show the part number. Now produce a new sheet or drawing of the flat pattern of said part and add a parts list to that drawing. Under the latest update of fusion the part number will be the same. Now compare the drawing of each part. Are they remotely the same in any way?