I have been playing my 12 string guitar for about 40 years and my fingers seem to have recently got fatter, making it harder to hold down individual strings without touching the adjacent ones. Apart from the different sound, I wonder if removing 6 of the strings, making it like a 6 string guitar, would be a way around this problem.
I think it would be better not to do that. Your guitar's neck is built to have the tension of twelve strings. By removing half of the strings, you'll be taking out about half of the tension that needs to be there to keep the neck straight. This could even go as far as changing the neck's shape in an unwanted way.
A luthier probably wouldn't be able to adjust the neck enough to compensate such a drastic change. Although you can always ask!
You can remove half of the strings and convert your twelve string into a six string with very wide string spacing. But I will tell you a few reasons why you might not want to.
But first, here is what you should know if you do. Be aware that you will reduce the overall tension to some degree with fewer strings. You will probably notice that the relief in the neck (slight bow away from the strings in the center) will flatten out some with only half as many strings.
You may like the lower action but if all the relief comes out, and you have a straight neck or a slight back bow, you will need to adjust the truss rod by turning it counter clockwise to loosen the compensatory tension applied by the truss rod. Otherwise you will have strings buzzing on the frets.
The other thing you need to be aware of is that the depth of the nut slots may not be optimal for a standard six string set of strings. String gauges in a twelve string set are much lighter for each string than on a six string set. You won't get much volume using only six of the strings from a twelve string set. If you beef up your string gauges by using a six string set (even extra light) they may not seat properly in the nut slots in which case you might need to have a qualified luthier enlarge the nut slots.
On the subject of nut slots - the string spacing both at the bridge/saddle and at the nut, is optimized for six pairs of strings. To optimize the spacing for six strings, might mean splitting the difference between each of the pairs. To do that on the nut side of the scale you could have a new nut made. On the bridge side, I don't believe the bridge pin holes or string slots (if you have a pinless bridge) can be so easily adjusted. So your string spacing might be a little off center with the neck which might not only look funny - but might require some adjustments in your playing technique (with 12 strings you were aiming for the center of the pair - now you are aiming for the center of one of the strings).
Also - if you don't readjust the string alignment and you select the top of each pair for your six strings and maintain that process for all six to keep string spacing relatively consistent - your high e string will be farther from the edge of the fretboard than normal. If you choose the second position of each pair to remain, then the space between the low E string and the edge of the fretboard will be larger than usual.
Here is another reason the conversion from 12 to 6 string may not make as much sense as just getting a wide nut six string.
A twelve string guitar will usually have more mass in the neck to accommodate more strings and more string tension. The bridge will be a little larger and heavier as will the headstock. All of this adds up to a heavier guitar than a six string.
You might want to consider a six string guitar with a wider nut width and wider string spacing. For example the standard string spacing on a Taylor Guitar (I own two) in the 300 series and up, is wider than what I find on most other guitars. In fact you can get an American made Taylor guitar with either a 1-11/16th inch nut width, a 1-3/4 inch nut width (standard on 300 and up), or an even wider 1 -7/8 inch nut width which is as wide as most and even wider than some 12 string guitars. Here is a link to a 414 CE on Taylor Website showing standard 1-3/4 inch nut width and listing the other two as options: 414ce Taylor.com
Of course a nylon string classical guitar will have wider string spacing but that is not an option if you want to play steel strings. They are not usually made to compensate for the higher string tension of steel strings.
Bottom line - You can make the conversion if you must, bearing in mind the caveats and adjustments suggested above. But if it were me, I would shop for a six string with one of the wider nut width options. Good luck.
If you remove one of each pair of string, two things may happen:
The action will almost certainly get lower, which will make the guitar easier to play but may cause fret buzzing.
The intonation may be off on the higher frets. To test the intonation, play the harmonic at the 12th fret and compare it with the fretted note. The two should be the same. Obviously do this before removing the strings as well as after. In either case, the solution (where possible) is to adjust the bridge.
Both these issues should be apparent immediately when you remove the strings. If you have either of these issues, you should probably abandon the idea, sell the 12-string and buy a 6-string, because asking a luthier to re-work your guitar to solve these issues doesn't make sense. Otherwise you're probably ok.
I don't buy the idea that you can do permanent damage to a guitar by reducing the tension overall. After all, when guitars are made they have no strings on them. There may be some instability in the tuning and intonation in the first month after a drastic change in tension though. On the other hand, I would never leave a guitar with strings only on one side of the neck, as this could well warp it. That said, proceed at your own risk.
If your fingers are too big to hold individual (pairs?) of strings without catching the adjacent strings, then it won't make any differencde when you take off the extra six strings. The spacings will still be the same, and the problem will still be the same. Best bet is another guitar, with a slightly wider fingerboard.
I just did this on a vintage solid rosewood Yamaha 12-string. I simply do not understand the fear in so many answers here! Where are the people who say, "go for it, good luck!"?
For me, it worked great, though the nut may have to be filed a bit to lower the thick gauges. The sound? Incredible. My guitar sounds better as a six-string than it ever did as a 12!