Yes! If there’s an old question with an incorrect or outdated answer, then you are more than welcome to post a new answer with the correct information. You can also bump an old post to add new and relevant detail to it. You can also make a new answer if you simply think you can write a better one. Anything that makes the post more useful to readers is allowed and encouraged. (Old threads still get many, many visitors, so it’s important they have good answers, too.)
‘Necroposting’, as it’s commonly known, is a taboo on forums because people view old conversations to have run their course, and don’t like seeing them in the navigation. But PokeBase isn’t a forum – it’s a collection of solutions to problems that people might have for many years down the line. Keeping them updated is absolutely worth our time.
The only time you shouldn’t post on an old thread is if your post is a duplicate (i.e. a repeat of what’s already there), off-topic, or if it's plainly uninformative. There’s no need to bump a thread to say ‘thanks’ or ‘woah that’s cool’ – just leave a vote. Also, we recommend posting a brand new thread instead of adding follow-up questions to existing posts.
Note that if you're adding new detail a question (e.g. adding info from newer games), then you should include any old information that is still valid, so that there can be one complete answer. You shouldn't have to look at multiple answers to get the complete answer.