I wouldn't worry about safety - although the aircraft are old, BA has a very good safety record and the aircraft are still held to the same standards.
I have done WTP to LAS on a 747 and I can't really reassure you about the quality of the cabin or the entertainment, but the plane won't be falling apart around you, noisy, or bumpy.
The unrefurbished 747s also have a smaller business class than the most recently refurbished ones, which reduces the chance of an upgrade on the day.
I was a BA silver at the time, no chance of an upgrade, but I would keep looking because all WT+ tickets are upgradeable so if you do have avios, or can get a cheap upgrade at the airport, it'd be a good use of miles because a flat bed on an overnight flight is a truly wonderful thing.