Upvote:3
The eVisitor visa is valid from the date it is granted for twelve months, and you can't stay for longer than three months during a visit:
You can stay in Australia for up to three months on each visit within 12 months from the date your eVisitor is granted. (source)
So if you have a eVisitor that was granted in October, you are okay to change your travel dates as long as you're not going to stay longer than three months.