My experience is that after a fixed contract expires you go onto a rolling month by month contract and have two choices:
1) You can stay on the month-by-month which gives you flexibility.
2) You can give them a call and suggest that you are considering whether to stay with them now that your contract has expired and they will offer you a cheaper rate, better bandwidth etc in return for committing to a new 2 year contract. Obviously if they don't offer you a significantly better deal than your rolling contract (or the market generally, or if you just want to keep your flexibility) then there's no point committing to a new 2 year deal.
I've never come across the 'auto-renew' thing personally, with either PCCW or Cable TV in the past but maybe I've just been lucky.