Today I answered What is the maximum iframe width of a facebook page? (July 2011), but before posting an answer, as always, I searched for duplicates. I found Facebook - Maximum iframe width? with an accepted answer, which was correct the day the question was asked. Because of the changes in Facebook platform, that answer isn't correct anymore, and I think the OP of the question I answered also found that question, and asked a new question. This could be seen by title that he put to his question, "Maximum iframe width? (July 2011)". That's why I answered his question with information relevant today.
What is a common practice in cases like this?
Should I edit old question, adding correct answer, should I indicate somehow that there a question with relevant answer?
Should they be merged in one question?