I am afraid that it is impossible. Upgrade the output power of a power supply means enlarge the output current. So you need to do (but no only) is as below:
1. check if the Rectifier bridge is enough driving the current that you want, if ont, you have to enlarge the value of them.
2. check the ic to see if it is big enough to drive a 400W solution
3. you have to chage the transformer, and take a bigger one. This is the big problem because there is no enough space to build the bigger one on the original PCB...