Where is the problem? If the enviroment allows a lens change and you can not lay the camera on a flat reliable surface:
- Camera with lens on the belt around your neck.
- Lens ready (i.e. upper side upwards) for change in the right hand
- Unscrew rear cap with left hand, store cap in palm
- Grap the camera lens on the camera with left hand
- Press release button with index finger of right hand
- Unscrew lens with left hand
- Now you have one lens (and the rear cap) in the left hand and one in the right
- Set the lens in the right into the camera mount and turn it until it is fixed.
- Take the lens from the left hand into the right and set the rear cap onto it.
Do not hurry! Be carefull not to drop a lens!
Mind you: I am not responsible if you drop the lens. Try it above very soft ground first.
My personal experience is, that a lens like the 16-85 is completely sufficent for travel photography and you rarely have to change it.
The better alternative to lens change in the field is a second body, which does not has to be a DSLR. Take a good compact with you for the rare occasions when you switch to the 55-300.
Did I mention I am not responsible, if you drop a lens?
EDIT:
A saver way to change your lens is like DeadJohn described. A Spare rear cap comes handy then.
And like UncleVanya said, one should avoid lens changes in field. If there is any wind you will get some dirt into the camera.