Hello, there!

Upping a thread by only posting "up" is allowed in only a few case. If it is really useful, some people do it (i.e. if a Mod gets asked to unlock a thread, some write "up" to avoid an early system based locking). But normally it is better to write something that belongs to the topic than to say just "up", because this can help to re-start a discussion much more than just "up".

So my suggestion is - in the case that you want to re-start a discussion - to add something useful to the topic. After some time of lurking you'll learn when you might be allowed to write "up" and when you're not. But this will be different from board to board. So if you're not sure, don't hesitate to ask the Mod of the current board.