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You're facing a common challenge when upgrading motherboards. The ASRock-z390AS-4S-IB socket is compatible with an i7-9700K, but switching to an i9-9700K on the same LGA 1151 platform isn't straightforward. The compatibility depends on specific features like chipset support and power delivery. Since the parts are from the same series, it's possible, but you should verify the exact model numbers and confirm with a reliable source before proceeding.
It seems likely you have the correct BIOS for your i7 9700K. The i9 9900K and Z390 chipset should work fine, though I’m unsure if the VRM will handle it properly.
Your motherboard belongs to the 300 series chipset, making it suitable for 8th and 9th generation components. The i9-9900K with eight cores fits this setup. LGA 1151 can be a bit confusing since similar models appear on 100 and 200 series boards such as B250 and H170, though those are designed for older generations.
This processor is essentially the same as yours but with enhanced capabilities through hyperthreading. Performance gains might reach around 25-30% depending on your software support. Currently, the i9-9900K remains prohibitively costly relative to the benefits you could achieve. None of the newer Intel offerings justify the price on most platforms. I recently switched from an i7-8700K to an i9-10900KF, but the difference wasn't significant. If you decide to upgrade, consider moving to the Ryzen lineup and a 5900X—even the 5800X provides only modest improvements. The 5900X offers more flexibility for various applications due to its higher core count.
Isn't the KF also capable of overclocking? Actually, the F indicates the GPU is turned off, while K means it can be overclocked. You won’t gain much more performance from overclocking, but the i9 might not offer a significant boost unless you can leverage the extra cores.