well, in that threar, what it truely sais is that you can choose, once you get a power, any of your availabe powers to upgrade. After you speak with you trainer(and if you are lvl8 or above) the training window rise; on the bottom of it (if you have one of the above diciplines selected) will appear all the powers you ´ve got from that discipline, and you ´ll be able to put some of your power points to rise any of them.
Remember that you only can rise powers if the discipline is ranked enought . ex: you need the discipline to be on lvl 19 so can you get the last power(10) and rise it to 5 (max) and to do al this you ´ll be needing a 37lvl pj
Hope it helps.