Limits
Smokey is right.
I had some questions about Vmo some years back, given I was flying an old aircraft and in decent I always pushed red line.
In this case, VMO testing for this aircraft was to take it 50kts past and do a multiple g pull up, I assume to simuliate the pilot waking up or something and having to avoid the mountain he was descending into.
Another Vmo limitation has nothing to do with excessive drag or engine limitations but like on the Citation X, where it starts to turn left when it flies too fast. Remember the SJ30 crash during testing, pilot didn't reduce power, plane rolled, and that's all she wrote, because he went REALLY fast after that..
The point, the limits you are given are no where close to flight tested capabilities, but the plane might do funny things past the limits that you are given.
Some planes act funny closer to those limits then others. Talk to Falcon pilots and they have no problem taking the wing past mach. Talk to Lear 35 plots, and the wing starts buzzing, maybe even tuck only 10 kts past Vmo.
I suspect, that your 747 could fly to what is known as absolute altitude, and if I remember it's where the plane will not climb more then 100 ft/minute. So if it's cert for 45, another 6k shouldn't be a problem, but what about pressurization, coffin corner, flight characteritics, compressor issues, bleed air, ect.
Food for thought.
SSG