This PC Access version 4.00 Disk 1 has already been upgraded to serial number yyyyyyyyy. You can only upgrade a version 4.00 Disk 1 a single time.
Once you have used a PC Access version 4.00 Disk 1 as a proof of ownership in a PC Access 4.20 upgrade edition, you cannot use this same version 4.00 Disk 1 again. Each Upgrade edition copy of PC Access 4.20 you purchase must use a separate PC Access version 4.00 Disk 1.