Not sure where you heard that.
The 25 shared a powerhead with the 20 horse (about 25 cubic inches of displacement), not the 30 (39.4 cubic inches of displacement which was shared with the twin cylinder 40 horse). So you can "detune" yours a bit but can't bump it up enough that you will notice.
In small outboards the max horses is pretty well limited to the displacement (on a 1:1 basis), and the 25 is already at the max.
On models that can be converted 9.9 to 15, 20 to 25, 30 to 40 etc, the cost is usually significant. It can encompass changing the carb or reed valves/plates, altering or changing the exhaust plate/tube etc (big bucks compared to the price of the motor).
AND, if you live in the U.S. it is illegal to alter the horsepower of the motor (unless you have a racing exemption - thank the EPA for that law). Ok, that wouldn't stop alot of guys, but should an accident ever occur and things were investigated you can be sure that the fact the motor was illegally altered would weigh heavy on determining fault (so something to consider for US residents).