That'll work. Then let your current plan expire, whereupon you'll start a grace period. Then log back in within the grace period, choose your desired new plan, and (if you wish) reactivate auto-renew.
....Or don't worry about it because you've only made a S$3.80 "mistake." That won't be the most expensive mistake you'll ever make.
Ask them, but let's assume Eight says no, that they don't allow you to change your mobile number. And that you have wallet credits with Eight that you want to preserve. (If you don't, just get a new Eight line. Maybe even with a new line promotion.) I think this workaround should work...
1. Get a new line with another carrier with a new number. Don't choose SIMBA (since they make things a little difficult). Zero1's Starter Prime (S$7.06) is probably a good choice for these purposes.
2. Head to an Eight service center with your ID and ask them to port your new line (from Zero1 for example) into your existing Eight line. Please note that you will lose your existing Eight number when you do this.
Also please note that there could be restrictions (or at least fees) if you obtain a "golden number" then try to port that out. And it's possible this won't work if your current Eight line was ported in, but for S$7.06 it's not expensive to try. (I think it probably will work, though. There are people who want to reduce the number of lines they have, so a second port in should work.)
By the way, if you're changing numbers because you're trying to avoid someone, there are probably better ways to do that.