You might not need to change the piston, it could be only valves or rings. Wese, it is best that you keep the engine healthy instead of maintaining oil level which can cause a fatigue at any time, specially for long tourers.
Also, are you sure it is 25,000kms? Because i ride almost 30kms a day and including my trips it will be more than 15000kms in one year. Do you not use your bike in routine?