Can high-bypass turbofans cruise at high altitude? They have talked for years about re-engining the B-52 with something more modern than its TF33-P-3/JT3D turbofans, which are fairly low-bypass (1.42:1 for the JT3D).

Modern engines are much higher bypass - like the GE Passport at 5.6:1, similar to the CF34 - in the same weight class and thrust as the TF33.   A larger engine like the LEAP has an even higher bypass ratio, e.g. 11:1. 

Now, the B-52 has a published operating ceiling of 50,000 feet.  The 737, A320, Bombardier RJs etc. have a ceiling of 41,000.   

However the KC-135s, re-engined with semi-modern CFM56 high-bypass turbofans, have a ceiling of 50,000 feet.  

So I am confused. Does the bypass ratio have an impact on the practical ceiling of the aircraft?  Could the B-52 fly to current spec with engines like the GE Passport? 

A side question... the TF33 claims a specific fuel consumption of 0.56 lb(lbf-h).  I can't seem to find that figure on the most recent high-bypass turbofans.  Is it inapplicable to high-bypass turbofans?  If not, is there a viable way to figure what a modern engine would do to the B-52's range?
Yes in principle, but some modifications are advisable.

What limits the maximum operating altitude of a jet engine (besides the thrust needed to climb up there) is the length of the combustion chamber and the absolute pressure of the air entering it. Since the atmospheric pressure drops with altitude and the compression ratio of the compressor stays constant, the absolute pressure in the combustion chamber drops with altitude.

For combustion to take place, first the fuel droplets injected into the air stream need to evaporate. This is made easier by higher temperature and pressure, and the lower both are, the more residence time of the fuel-air mixture (caution, PPT link) in the combustor is required for good combustion. Longer combustors have higher pressure losses and weigh more, so engine designers try to limit their length.

Just throttling down can already cause the ignition to stop at high altitude. If a flame-out occurs, the engine cools down quickly and a restart at a lower altitude might not be possible. Turbofans suitable for high altitude operations use special measures to stabilize the flame. When they are in place, there is no reason the B-52 can't continue to fly at 50,000 ft.