"There's nothing stopping you from interacting with others on IRC with your new handle."
Common sense, if you mean ICLY interacting with others. No mechanics prevent interacting with others on IRC with any name that isn't attributable to another player. However, That new character doesn't exist yet, so you shouldn't play that character ICLY yet, regardless of what name you use on IRC. Is that likely to be enforced at all? Probably not. Is it likely to be extended to an option for Forum Posting and DMs? I highly doubt it, but at least that suggestion isn't literally a restricted topic.