I bought a secondhand Mele ukulele recently. It's the first solid uke I've owned (with solid mahogany sides and back, and a solid Koa top), and I love it. The double sound hole provides a wonderful rich sound for such a small instrument.
What I don't love is that the previous owner had "upgraded" the original friction tuners with geared machine heads, installed upside down because they wouldn't fit on the headstock the right way up. After a couple of weeks of trying to adjust to tuning it backwards, I decided it needed modding.
Gotoh's ukulele planetary tuners (UPT) seemed like a good solution - still easy to tune because of the of 4:1 ratio of the internal planetary gears, but with the look and feel (i.e weight) of traditional friction tuners. I got mine from The Ukulele Site
Installing the new tuners required a couple of changes to the headstock: first removal of existing tuners and filling in the screw holes, as well as reaming out the holes to fit the UPTs.
I used a multi-copter propeller reamer to enlarge the holes because I don't have a proper wood reamer, which did the trick (though it chewed up the insides of the holes a bit, these won't be visible once the tuners are installed).
To fill the holes left by the geared tuners, I filled them with sawdust mixed with superglue. Then I used a mahogany colored wax crayon (that I've used to repair furniture scratches) over the top to disguise the holes further.
After applying the wax crayon, I buffed with a soft cloth so that only the wax in the holes remained. I used an awl to poke the holes for the teeth that hold the tuners in place and screwed the tuners in.
Warning: Awls are sharp, hands are soft.
New tuners installed and ready for restringing!
Things that I could have done better:
I scratched the front of the headstock slightly during this process. I probably should have taped some masking tape over the surrounding wood to avoid this. The scratches are pretty small so I can probably buff them out.
I should have ordered the longer sized tuners. These barely fit, so I've decided to leave the washer off so that the screw-in bushing gets a better grip. The tuners it came with didn't have washers either so it's not a big deal, but it would have looked better with the washers.