i have
MSI MPG Z370 Gaming Plus
or maybe 270 - i should check.
are they not the same chipset?
7th Gen Core i7 like the 7700K you have, is not compatible with z370 or z390 chipsets which is required to upgrade to a 9th Gen Core i9 9900K**.
(z370 Motherboards require a BIOS update to support 9th Gen CPUs, z390 natively support 9th Gen)
7700K is only compatible with z170 or z270. So your motherboard would most likely have a z270 chipset.
Despite using the same 1151-pin socket, changes were made for 8th Gen and 9th Gen CPUs, which means that eventhough a 7700K CPU will physically fit in the socket on a z370 or z390 motherboard, the pin layout is different, so the CPU won’t work.
So as it has been mentioned already,
no matter if you are upgrading to a 9th Gen CPU like the 9900K or one of the new 10th Gen Intel CPUs, you would have to buy a new motherboard as well.
When it comes to price vs. performance, upgrading your 7700K to a 9900K isn’t really worth it in my opinion, since you’d need to buy both the CPU and a motherboard
- and if you want to upgrade to a new Intel CPU, it would make more sense to choose a 10th Gen CPU and motherboard as the upgrade path.
The 9th Gen motharboards don't have any planned future upgrade options as far as I know - so in a way, you could say that the platform is already "dead" with the arrival of the new LGA1200 socket and 10th Gen CPUs. So there's no reason to buy a new motherboard for that generation.
EDIT :
**Technically, a B365 chipset can also be used, but personally I would get a z-series motherboard, preferably a z390 chipset with decent VRM design specifically when it comes to the 9900K because of the high power draw, which can be a problem on some z370 series motherboards, depending on the quality of the board and its VRM design.