Most motherboards that use socket 775 are able to be upgraded with more advanced CPU than what you currently have installed, but when you try sometimes to do a replace to higher CPU it does not work probably and PC fail to start.
Not the same when you put a lower CPU than what you currently have, it will work probably, but no one will ever try to lower the PC speed by replacing CPU with lower one.
Before you try or start to upgrade a CPU in a motherboard first you need to lookup online for the manual of your motherboard and browse the CPU Support page. If the CPU is supported you will see a Bios version the board must have in order for CPU to function.
For example when we talk about Socket 775 like GA-945PL-S3 and browser CpuList , you will see what LGA775 CPU this board can support and also the bios motherboard need for the CPU to work. (N/A mean not supported).
Upgrading the bios need to be processed in a PC that function and working, you can not upgrade a bios in not working PC. So before you change the hardware make sure you upgrade motherboard bios before installing the faster one.