I would advice against getting as big as possible a PSU, since that wastes money and power. Quality is what you really want.
Anyway, if the 400watt has no shortcommings compared to the 350 watt (number of connectors, amount of power on each voltage, quality). and is in no way larger or smaller than the 350w. Then yes, you can replace the 350w.
But you should probably tell us what system it is supposed to run, and which powersupply model you have and which one you are planning to use.