If you want to add more memory you have to ditch the old kit
No, you don't "HAVE" to ditch the old kit. The fact is that at JEDEC speeds, mixed memory stands a very good chance of working fine, especially if you add an identical part number. The majority of problems happen at XMP speeds and timings that are ABOVE the JEDEC standards speeds, which for Intel on Kaby lake is 2400mhz. Keep in mind though that SOME boards and chipsets might only support specific speeds, for H110 for example, that is generally 2133mhz.
That being said, it's correct that using mixed memory (And ADDING a stick, even if it is the exact same part number, IS the same as using mixed memory because there are never ANY guarantees unless you purchase ALL sticks together in one kit) can be a crapshoot but for the most part at speeds that are within JEDEC specifications most memory with the same specs will run together if there are not wildly different timings or voltages.
The recommendation is ALWAYS to buy a kit that contains the full amount of memory you wish to run, but for 2400mhz sticks I'd probably just get another identical part number. If it doesn't work automatically you can certainly make some adjustments in the BIOS to try and get them to work together or worst case scenario, return the new stick.
If you wish to avoid any possibility of that, then I'd just buy a full set in the amount of memory you wish to run and sell your current stick or save it for troubleshooting or use on another system.
These are all good reads if you wish to learn more.
*Resolving memory problems and setting up XMP/DOCP/AMP profiles
https://www.tomshardware.co.uk/ddr3-dram-faq,review-33220.html
https://www.tomshardware.com/reviews/ddr-dram-myths,4155.html#p1