The best time to renew your mobile phone plan would be after your contract expires. then u can renew your contract and get a new phone for free. Roadshows like IT show, PC show usually have better deals so you may want to consider renewing your mobile plan there. the $100 offset is for users who port out from other telcos and join the new telco with a new two year plan.

in most countries, all telcos have agreed on the basic cost to be charged to customer, hence all plans they offer are basically the same, just with different packaging. the 100$ is usually to lure u to move away from ur current telco, usually they'll give voucher to current subscriber nearing the end of term also to keep them in. Perhaps can try tell them u quit their service soon, maybe they ll offer u something.