How can I update instances of a text template after changing the master template?
Unfortunately, that isn't currently supported. Your text items will preserve the state from when they were first added. So you're stuck doing it manually. It's number 4 on the list of top requested features for the next update though and Apple seems to be aware of this site, or they've just been reading minds the last few updates- http://fcpx.tv/top.html 

One thing that has helped me though is that you can drag your new Motion titles on top of the old ones already on your timeline, hit "replace", and it should retain the original text of the old title.