If you are changing chainrings and cassette then you should change the chain too. Components tend to wear together. Depending on the amount of chain stretch the existing chain received, it might accelerate wear on the new components.
The gearing change you are recommending will give you a broader range of gears for more diverse conditions. The trade-off is that their will be a less smooth transition between gear changes. Its a good setup for varying terrain and speeds, but less efficient shifting if you plan to race it.