A: Most computers are BIOS based, and BIOS software hasn't changed much in its 30 years. It's responsible for the initial startup and initialization of the machine and devices, then passes control to the OS. Unified Extensible Firmware Interface (UEFI) is a replacement for BIOS and is used on newer machines, including ones.
UEFI is far faster, and more secure and feature rich than the traditional BIOS, so many people might want to upgrade their machines to a UEFI-based system that enables such features as Secure Boot.
However, replacing the motherboard in a machine with a new motherboard that supports UEFI will cause some problems.
First, your processor might not fit in the newer motherboards that support UEFI. For example, I had a Core i7 socket 1156 and the newest Core i7's are socket 1155 so the new motherboards are socket 1155 based typically (or socket 2011) so there is a strong chance you might need to replace your processor as well. For most systems the motherboard and processor are a large part of the expense of the machine.
Aside from hardware, however, there are other issues. A BIOS-based machine must boot from a MBR (Master Boot Record) format disk. A UEFI-based machine must boot from a GPT (GUID Partition Table) format. This means your existing Windows 8 boot disk won't work in a UEFI system unless you enable compatibility mode in UEF, which means you lose many of the benefits of UEFI in the first place!
You can convert a disk from MBR to GPT but that would require backing up the content and restoring, modifying the Boot Configuration Database (BCD) and fixing other items such as creating a new MSR (Microsoft Reserved Partition).
Even if you get the disk to work you will likely face differences in storage drives and chipset configurations that might cause a problem.
Unless you have some hugely complex Windows 8 deployment, it's probably better to just reinstall the OS on the new hardware. Windows 8 is so new, if you are considering making the move to UEFI, do it sooner rather than later!