Amp & Speaker matching

The higher the impedance of the speaker the better the match with most tube amps. The "natural" output impedance of tubes is normally high and tubes don't dump current as easily as most SS amps.
The answers to your specific questions are, the output by the speaker will increase, and not be reduced. Niether the tube amp, nor speakers, will be damaged by the relatively high stated nominal impedance on your speakers.

Not sure I agree with Larryi.

The output transformer in a tube amp reflects the load impedance back to the the tubes. If you use speakers rated at a higher impedance than the output transformer impedance things will be ok. You will just get lower output.

If the speaker impedance is lower than the output transformer impedance then the tubes will draw more current than if the impedances were matched.

Whether or not this is a problem depends on the amp's design limits. Consequences range from none to damage such as blown output transformer, blown tubes, or worse, depending on amp and the amount of the impedance mismatch.

Check with your amp manufacturer for advice.

R Burke,
In my opinion the higher the speaker impedance the better for tube amplifiers,i.e. easier load to drive and generally less distortion. You should not have any problems.Low impedance speakers can be a 'potential' problem for some tube amplifiers.

Manley and VTL set their output transformers to approximately 5.5 ohms so their amps will perform well with 4-8 ohm speakers.

If you run a 16 ohm speaker on an 8 ohm tap, you may get around a 3 decibel loss in volume. On some amps, this can be a risk. It depends how they wound their transformers, and to what impedance they're loading the tubes to, plus other design characteristics in the amp. I would ask first to be safe. Some amps may be fine this way, others won't be. When the speaker impedance gets too high, it could cause the output transformers to arc inside. The worst is no load at all.