The reduced price depends on the previous version you may own, if you had the 64 bit version (P3D v4+), or if you had the 32 bit version (FSX/P3D V3). It will be automatically applied when you login with your regular customer account.
Only the Base Pack featuring the MD-82 is a payware upgrade. In case you also have the expansion MD-83/MD-88, it’s a freeware upgrade available in your history order. Look at the download details of the V4 product purchase order.