1>
Inspect you system and mke notes about the jumpers, cables and CMOS settings. Be prepared to restore the old system in case you have problems.
2>
Remove the Primary Slave from your CMOS setup. Just set it's entry to NONE. Many BIOS settings have an AUTO setting but using the AUTO setting will cost a few extra seconds during bootup especially if you don't remove the drive. Also if you leave the drive in, the Bios may find it and stop the bootup after a long pause to give you an error message.
If you set the drive's entry to NONE you will probably be able to leave the old drive connected. I would only do this If I got stuck with steps 2 or 3.
I don't bother with the seperate Auto Detect Hard Drive sections of the Bios Setup program when I remove a drive I just change its entry to NONE. Its quicker and easier. If you use it, you must do the other steps first.
3>
Check the jumper settings for you Primay Master drive. Some models will not need any changes. Others require a different jumper setting. The details can be found in the documentation that came with the drive. They are also on the labels of many recent drives and on the manufacturer's web site. Incorrect jumper settings cause problems.
4> Disconnect both the signal and power cables from the drive. I always like to have a drive on the last connector of the cable to minimize reflection / termination problems but this may not be necessary.