Tags are for questions not answers. This is unlikely to change.
What we are trying to do is categorize questions to try to get them the best answers, and make them findable and relevant to people searching. Adding tags to answers will really just confuse this attempt.
If your answer needs to specify something that would be in a tag (such as which system it applies to) do so in the text of your answer. This is part of the problem with allowing multiple potentially conflicting tags to a question (3.5 and pathfinder are the same a lot, but I'm sure they differ in some big places, dnd3.5 and 4e are very different, but the tags aren't mutually exclusive).
You're issue here is that the tags were changed after you answered. That's not your fault, but it doesn't require a change to the fundamental tag system. It either requires you to edit your answer to be more inclusive of the new tag or the OP to just be content with the answers as the question was originally tagged (including removing the new tag), or the situation staying as it is and there being a bit of confusing wrt how things work. It's up to you and the OP on how to resolve this, but it's a fairly rare case and I don't think it's worth changing something fundamental to the system over.