Hi,
I have searched high and lo but could not come to any conclusion.
I want to power my Arduino Uno using an external power source. A 9V battery is possible, however, it is quite troublesome to solder and wire everything. So a simpler method I thought of is to use a Power Bank (aka External USB Battery Pack).
Is that possible to power the Arduino Uno consistently? Thanks! 
Yes.
FWIW, I have a EZOPower 5000 maH USB dual charger and I used it to power my Uno R3 and it ran for about 4 days doing the blink program.
If you want batteries, these things work perfectly and last a lot longer than 9V battery (which have about the lowest power per dollar of any battery).
You can power then with one/two/three AA/AAA/C/D batteries.
What do you mean by "switch consistently"?