Firt of I'm not a 100% sure this is the right site to ask this on but here we go.
So I will have to buy a new phone in the near future. With this I'll most likely get one with USB c instead of now micro USB.
I have quite a few cables for recharging (1-3 at home, at work, and one for my powerbank) all of those are micro USB and still work fine, some of them are long cables so they weren't that cheap.
My question now is: is there any downside to buying adapters and sticking them on my already owned cables or should I have replace them? (clearly not all of them as I still have other stuff that charges with micro USB)
Personally I prefer cables without adapter. Especially when it comes to charging the adapter may reduce the maximum charging power (because it works like a resistor).
If you use an adapter I would strongly recommend to use an short adapter cable, not a small fixed adapter with a plug and a connector in a small case: The adapter without cable effectively extends the length of the plug and therefore also the force you apply onto the USB port on the phone's mainboard (because of the leverage effect). Hence using an adapter can decrease the lifetime of the USB-port.
Therefore I would replace the one or two most used cables for now. Others can still be used with adapter. In some months you know what you prefer and if it makes sense to buy more USB-C cables.