You can upgrade or downgrade your plan anytime, without penalty. However upgrades and downgrades behave differently.
When upgrading a plan, the upgrade takes effect immediately. Usually people are upgrading because they have run out of sessions and need them right away. Your credit card will be charged when you choose your upgrade, the date you upgrade will become your new subscription renewal date, the new sessions will immediately be added to whatever sessions may remain from your previous plan and all will be available for the remainder of the new subscription period.
When you downgrade the plan change will not come into effect until your subscription is next due to renew. We do this as people are usually downgrading as they are anticipating a quiet spell, but we want you to have the opportunity to use all of the session allowance you paid for in your current subscription period. When your subscription renews on its next renewal date, the plan will downgrade, and you will be charged the amount for the new plan.
Slush credits are not affected by upgrades or downgrades.