For paying users:

- Users have the option to upgrade to any plan, and the price difference will be charged accordingly.

- Upon cancellation, users will continue to enjoy all the features until the cancellation date, after which they will revert back to the free account.

- Once a subscription is canceled, users won't be able to upgrade or start a new subscription until the cancellation date of their last plan.

For users on a free trial:

- To start a new subscription, card details will be collected, and a 7-day free trial will be initiated. After the trial period, the user will be charged.

- During the trial period, users won't be able to change their plan or upgrade.

- However, they have the option to cancel their current trial and start a new subscription with the plan of their choice. To prevent abuse of free trials, a user will be able to start only a 3-day trial if they already used up a trial.

- Upon cancellation, the trial will be immediately terminated, and the remaining trial period cannot be used.

Regarding common upgrade problems:

- If a user cancels their current subscription, they will need to wait until their previous plan expires before starting a new subscription.

- An upgrade will fail if card details have already been provided but cannot be charged.

- Coupon codes cannot be applied to trial plans.

- When a user cancels a plan with a free trial and starts a new subscription, the free trial period will be reduced to 3 days.

- During the trial period, users are unable to upgrade or change their plan. If they need to do so, they must cancel their current trial and start a new one.

- Upgrades do not come with a trial period and will be charged immediately.