Why would I want to change gears? If you were riding a bicycle, you would want to shift to a 'lower' gear when going uphill, and a 'higher' gear going downhill.
When rowing, going against the wind (or current) and/or rowing a heavy displacement boat (thanks to Brian M. for suggestion to add 'displacement' factor) is 'uphill' (use a lower gear) and rowing with the wind or current with a light boat is 'downhill' (use a higher gear).
Let's define "gear" as it's used in rowing: Gear is the ratio of the outboard length of the oar to the inboard length of the oar.
The outboard length is measured from the tip of the blade to the pivot point on the oar at the oar lock and the inboard length is from the pivot point to the end of the handle.
My current set of oars are 8' (244cm) with an outboard length of 67.5" (171cm) and inboard length of 28.5" (72cm), resulting in a gear ratio of approximately 2.4 (171/72 = 2.375).
Koti, in the site referenced below, suggests the 'optimum' gear ratio is 2.5 to 2.7. If I moved the location of the pivot point closer to the handle by 1.75" (4.5cm), the gear ratio would be 2.6, right in the middle of Koti's optimum gear ratio.
So how do we change gears? Six ways:
1. Have two (or more) sets of oars with different gear ratios for different conditions. Long distance 'ocean' rowers carry multiple sets of oars, not only for safety, but also to deal with different conditions.
2. Move the locks. I've never seen this, but it certainly is feasible to have the oar locks mounted on blocks which could slide in or out and lock into position.
3. Move the collars -- the issue is that collars typically are permanently attached to the oar and can't be moved, although in competition rowing, the collar, also called the 'button', is moved to change gear ratio.
4. Slide the oar in or out on the lock. This works for a few minutes, but soon the oar slides out (butting up against the collar and thus to a higher gear).
5. Make oars with longer handles and move your hands out (toward the oar blade) for higher gear and in for lower gear. My oars have handles 5.5" (14cm) long. I can definitely feel the difference in 'gear' with the approximately 1.5 (3.8cm) inches of hand movement available on these handles.
6. Add a small jam cleat (Duckworks Jam Cleat 'SD-002040') and 3 feet of 1/8 non-stretch line as shown the photo below. Tie a tight loop around the oar loom just outboard of the cleat, wrap the standing part of the line around the oar lock and back to the jam cleat. This provides infinite and quick adjustment.
Two other advantages of this 'gear shifter':
1. You can let go of the oars and they will stay in the oar locks while you take a photo or drink some water.
2. You really don't need collars (unless you are a "belt and suspenders" person).
Measure your own current gear ratio and add the 'jam cleat' adjuster to increase your rowing efficiency.
Comment below with your experience in using 'gears' when you row. I'd love to hear from you.
For a detailed discussion on oar length and gear ratios, see "Optimum Sculling Oar Length" http://koti.kapsi.fi/hvartial/oarlength/oarlen.htm#top
For more on the "Physics of Rowing", see http://www.atm.ox.ac.uk/rowing/physics/index.html