Because looking at the parts with prebuilts isn't enough as identical parts can have different engineering specs. If you don't want to look it up to get the proper parts, then you may face issues that you can then ask forever and never get right answers because no one will be able to tell that your parts were wrong.
If you followed what I outlined, you'll be fine. Find your HP model number (usually ends in ABA) and look it up on the hp partsurfer web site. That will list every single part that was used in your system. Check the coolers section and see the part numbers available. If there's one, great--you can simply swap processors and should be fine. If there are two, you will need to find out which one you have and will probably need to get the other one for proper cooling. You can still do the swap now, but will need to set the fans to 100% in the bios for proper cooling until you get the proper cooler. Once you do, swap the coolers, change the bios fan setting back and that's it.