The hardest thing about installing upgraded tuners is that you often have to enlarge the peg holes - not hard to do, but might need some patience. You can either:
a. plug the existing peg hole by gluing a dowel into it. The you just drill a new hole. You should use a jig or drill press to make sure you're drilling straight and accurately, OR
b. use a reamer or a circular file to enlarge the existing hole - chain saw files work well in my experience.
If you're looking for high quality and lightweight open geared uke tuners, I highly recommend Grover Ukulele 14:1 Tuning Machines - they're good for pegheads of up 1/2" thick, and peg hole daimeter they require is realtively small - 11/32" - so you don't have to enlarge the existing pegholes much from that of a friction tuner. They're not too expensive either! You can see them here:
http://www.stewmac.com/shop/Tuners/Ukulele_tuners/Grover_Ukulele_Tuning_Machines.html
For a thicker peghead, you might want to consider GROVER STA-TITES tuners for guitars - thay only in sets of six, but these are great open-backed tuners! They are heavier than the Grover uke tuners, but they work well on heavier bodies ukes. Note that the string post is higher than those on uke tuners, so you may need to have your strings longer than you normally do ot make more windings around the string post - that way the string will sit snug against the nut. If that doesn;t make sense, ask me and I'll atry to make that clearer. Anyway, you can see them here:
http://elderly.com/accessories/items/GTM5N.htm
Planetary pegs are heavier and also require a larger peghole - more like 10mm or about 3/8". So yo have to remove more wood from the existing pegholes to install these - not a big deal but kinda tedious.
I personally like Schaller Planetaries - they have a big knurled locking nut so you can easily adjust the turning tension without worrying about stripping a little screw head. But there are also lots of folks who like the 5-star brand. They are both excellent quality pegs.
An alternative is to get narrow shaft planetary tuners which only require a 9/32" peg hole - much easier to install because of that and you don't have to drill out so much of your peghead. These pegs work fine and have the same 4:1 gear ration as the other planetaries, but they don't work quite as smoothly as their more expensive brethren. But they work just fine, but they're not butter smooth when turning them.
In any case, I happen to have a brand new set of Schaller Planetary pegs and new set of the Narrow Shaft Planetaries too. I'll sell 'em at a good discount if you're interested.
By the way, how thick are the pegheads of the ukes you want to upgrade? That will impact what tuners you can use.
Hope this helps!