The way I see it, if you are a member of a private organization, including a political party, you are subject to changes made by the leadership. Any private sector business, charitable organization, etc can merge with others and change their names whenever they want, and if you don't like it you're free to leave. Political parties aren't a branch of the government, and requiring a quorum of the members is already greater limitation than most organizations have. Also, party registration does not affect one's citizenship status in any way, and I don't see a reason to be so upset about it.
People shoudn't be forced into a group that they didn't consent to joining. I don't see what the issue with having people manually change their registration is anyway. I should also point out that last time when PUP and Labor attempted to merge multiple people immediately rejoined Labor from PUP that didn't consent to the merger. I don't believe not voting should be interpreted as being ok with a merger.
