If they have similar rules to their current retail tariffs then...
You can 'upgrade' your tariff at any time.
You can only 'downgrade' your tariff when you are halfway through your contract.
You can only 'downgrade' your tariff one step at a time.
e.g. If you are on the £55 tariff, after 9 months, you can downgrade to the £45 tariff only. Then wait a further month, and then downgrade to the £35 tariff.