Can I change the functions of buttons and knobs to suit my preference?

It depends on your application.

[djay for iOS]
You can change them as follows.
1. Tap djay icon > Gear icon on the top of the window.
2. Tap [MIDI Devices] > [DDJ-200 Bluetooth].
3. Tap items in [CONTROL MAPPINGS] and change the setting.

[rekordbox]
Yes. rekordbox has a MIDI LEARN feature.
You can use it to change the buttons or knobs that features are assigned to.
For details, refer to the MIDI LEARN Operation Guide.
You can read the MIDI LEARN Operation Guide [here].

*You can't change them when using WeDJ for iPhone, WeDJ for Android or edjing Mix.