No, the part will not automatically change. If you want the part to change parametrically, you need to creat the new part within an assembly. I didn't know how familiar you were with the assembly mode. Start a new assembly, insert the turtle deck part. Make sure its position is "Fixed." Create the new plane as before where you want the new part to be. Use the create new part function. Select the new plane as the new part sketch plane and use the intersection curve feature as before to get the sketch profile. Extrude the sketch as before and you now have the new former base shape. You can cut holes and notches in the new former to get its final shape. Just be careful not the break the link to the turtle deck if you make cuts in the original profile. When your done, exit the "edit part" mode. The assembly now has 2 parts, the turtle deck and the former, and the former profile will then change as the turtle deck profile changes. If you change the transparancy of the turtle deck you will be able to see the former.