Hi @rachie,
Welcome to the Bunnings Workshop community. It is fantastic to have you with us.
Timber floorboards are often installed directly over joists, so if you are looking to have the new floorboards flush with the old, then removing the chipboard flooring and attaching the new floorboards directly to the joists is likely the best way to go.
Most timber floorboards are fine for installation over joists as long as they span at least 3 joists, but you would want to confirm this with the manufacturer to ensure they are fine with it and will offer a warranty on this installation method.
If you find that the old floorboards are a different thickness from the new ones, then you may need to either plane back the joists to allow for the additional thickness or install packing to account for the new floorboards being thinner.
The alternative is to install them over the existing chipboard, but this would create a bit of a step up that wouldn't be the most desirable. You could always dull the edge of this step by using timber trims such as quad, but it wouldn't be as nice a finish as a flush one.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
