I agree if it is fundamentally changing the nature of their code, but expanding an answer to include a Python 3 solution alongside an existing Python 2 one is something I've done myself and haven't experienced any issues with. IMO the most important part is to preserve and extend the original answer vs re-write it completely.

You don't have to modify, and I never do except to improve readability (read: eliminate or reduce horizontal scrollbars). It's not unusual to see a good contributor add an answer to an old question that uses newer syntax or version features. "The accepted answer is quite old but this question is the top result for 'javascript arrays' (e.g.), so this is how you do it in ES6."