Upgrading 2019 iMac with i5-8500 to i9-9900

Thou shall upgrade to the same chip Apple used in their i9 model your series uses iMac19,1 If you have a different series you’ll need to review what it offered.

Kidding aside, are you sure you want to do this? You’ll loose your warranty coverage! As the system is so new.

You run the risk of damaging your systems display if you don’t follow the proper methods in taking off the display as well as use the correct tools.

Frankly, I would put the money into upping the RAM 16 GB at least, and if you are heavy into graphical projects then 32 GB might be beneficial.

Drive storage is another area one can get huge performance benefit. If you currently have a Fusion Drive I would look at getting a high speed external drive (TB3 single SSD or for the max, a RAID’ed SSD) and set it up as the boot drive. TB3 is faster than your internal SATA drive and even the Fusion Drive setup too.

If you want to upgrade the internal drives that works too! But again I would wait until your warranty has expired before messing with the internals.