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...