Are you referring to one of Fido's Lunar New Year promotions? Are you also intending on upgrading your phone? As far as I am aware, those offers are only for new activations or hardware upgrades only. If you already have services with Fido, your only option to change to one of those promotional plans would be to upgrade your device as well. If you had not intended on upgrading your phone, that could be the reason you're not able to choose that plan.
Hope this helps 😀