If I have a question that I asked a long time ago (Over 2 or 3 months old), and it already has an answer that I have marked as the accepted answer, can I slightly modify the question so that the meaning of the question might differ? The reason I would want to do this is because I have another question that is similar to the first question. However, I think that if I asked it by itself, it could be seen as a duplicate of the first question. This second question could be added onto the first question and it would still be part of the same question. I am referencing This question. I would like to modify it to ask "what's the difference between bokeh, gaussian blur, and blurriness"
If the meaning is different, then ask another question. Generally, we're pretty careful about only closing a question as duplicate if one of the answers in the previous question directly answers the new question, so it is unlikely to be a problem.
Conversely, editing an old question to change the meaning removes useful information from the site and risks not being noticed as it is an old question.