There is no graphical user interface yet. But as I said [ DO NOT POST SESC LINKS #top]in this post[/url] (and also was mentioned by TryNinja in the thread you linked) there is the addwitnessaddress command.

It should also work in the graphical Core client, at least from 0.13 onwards (selecting Debug Window -> Console).

If you want to use only Segwit transactions from now on, what you can do is create a new, fresh address, convert it to "Segwit style" with addwitnessaddress, and move all your coins to the newly created address. If you want all your coins on Segwit addresses, you should then take care that you don't accept any transactions to "legacy" addresses (so create some more Segwit addresses if you want more privacy).

I read that also NBitcoin does the trick, but that is a library for expert users and programmers.