I always go for the book-early-cancel-later approach for flights that I am not entirely sure about. Booking last minute nowadays is almost impossible for reasonable prices. However when it comes to economy fares the fees for refunding are rather high, so I either try to book a cheap business flex fare (if available), change the dates rather than cancelling, or go for an award redemption - which can always be cancelled for just the 23 euro.

If your plan is to change origin/destination airports I would suggest to call them to determine how that works, first. Most likely there will be a split between fare and taxes, meaning that those will have to be recalculated separately. This also means in the worst case that the fare may be significantly cheaper, taxes higher - meaning you would still have to pay extra. I would not expect them to refund part of the fare. Taxes..perhaps.