The old question's answer is correct but no longer applies
Do you mean, "was correct" then? Or is there still an old version of Facebook around, where the old answer is still valid or useful? Because if not... Well, there's hardly any good reason to preserve the older answer then.
In cases where multiple answers to the same question can be valid for different users, there are simple ways to reflect this on the site:
- Have multiple answers, each describing a solution for a specific scenario.
- Have a single answer that describes multiple solutions, noting where each is applicable.
When starting with two separate questions, the first is easier to achieve: simply flag for moderator attention and suggest a merge. This will result in both answers being attached to one question.
Alternately, you can edit both questions to reflect the unique, specific circumstances in which their answers are valid. However, I suspect this would be wholely inappropriate for this specific instance, as "How could I have disabled Facebook's Chat back in July of '10?" is not a tremendously useful question.