Assuming it hasn’t been closed of course, and assuming that you’ve read through it and have something new to add to it? I’m talking about years-old threads.
But I believe it’s acceptable to start a new thread with a title regarding the new information and post a link to the old thread inside the thread for very old thread responses.
It’s no good on this message board, as we have no “delete post” option, but on other message boards that let you delete your own posts, you can perform what I like to call a “stealth bump”. Add your bump to the thread, then imediately delete the bump. Many message boards don’t move a post up or back when you edit/delete, so you get the effect of a bump, while keeping the signal to noise ratio in the thread low.