You can have several competing specifications, recognizing the old BTC balance and offering a new balance in a new spec. People (who can be) separate from the s

You can have several competing specifications, recognizing the old BTC balance and offering a new balance in a new spec. People (who can be) separate from the spec proposers may implement clients that connect both to the Bitcoin network and the alternates. Eventually, one (or maybe more) new protocols win out and trade move to the stronger protocols. BTC would be left as legacy and eventual discontinuation.