To be precise I think, as a soft-fork that narrows the consensus rules, it can narrow them and then expand them up to the previous limits without problems as the old implementations will be compatible and accept narrower or the same rules. Its hard fork only when expanded and backwards compatibility...

On your point actually, if a new version of the node software is released with narrower consensus rules and after that rules needs to be expanded again that will break that version. It won't be able to work any more.