Oddly enough, I just did almost the same thing last weekend on my wife's build. I swapped her 4690k, some cheap generic mobo, and DDR3 RAM for a 2600, b450i, and DDR4 RAM without formatting or reinstalling windows. It was for the same reason (time constraints) and I do plan to correct it this weekend when I have some time.
The positive - it works. All her programs load, the system doesn't crash, and it's noticeably faster than what she had. Windows is using the correct drivers for all but 1 component (see below) and it removed/disabled drivers for devices which are no longer present.
The negative - it doesn't work as well as a clean install. It has some occasional hiccups, and the mobo's built-in bluetooth doesn't work even after removing all bluetooth devices from device manager and installing the correct drivers.
Overall it's good enough to get out of a bind, but I am not planning on keeping it this way.
Edit: I did have to reactive windows 10, but it was a painless process.
Gigabyte x570 Aorus Pro Wifi
Sapphire Radeon RX 5700 XT
Crucial Ballistix Elite DDR4 3600
Sabrent Rocket NVMe PCIe 4.0 M.2 2280
Sabrent Rocket NVMe PCIe M.2 2280
Seasonic FOCUS Plus 750
EK-KIT Classic RGB P360
EK-Vector Radeon RX 5700 +XT RGB - Nickel + Plexi
Arctic P12 PWM PST
XSPC G1/4" Inline 10k Sensor, Chrome
EK-CryoFuel Solid Cloud White (Premix 1000mL)
Lian-Li PC O11 Dynamic (white)
Windows 10 Pro
Audio Technica ATH-M50x
NVIDIA GeForce GTX 970M
Samsung 850 EVO
HGST Travelstar 7K100
Microsoft Sidewinder X4
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