Hi, first message here.
I have a 450W power supply to substitute an old 250W one for my motherboard and PC.
The question is, since I am not an expert in power supplies, can my circuits take some damage if I use this greater W power supply than the originally designed one?
Your power supply merely makes the power available, the components in your system will only draw as much power as they need. You could replace your 250W unit with a 1500W unit and as long as all the right connectors are there it will work fine.