If you're trying to downgrade to a plan that's on a lower tier than the one you are currently on, you may run into an error that says:
Downgrade is not supported on this plan.
If you see this error, it means that your current Kayako setup wouldn't function the same way, if you downgraded your account. For example, perhaps you've added multiple SLAs, which aren't available on a lower plan. Or maybe you're supporting multiple brands, which is only an for option the higher tiers. Downgrading your subscription would render certain functions of your Kayako unusable, which is why you're seeing this error message.
If you do need to downgrade, you can check the pricing page to see what each plan includes. Before you can downgrade, you'll need to adjust your Kayako configuration to match the limits for your desired plan.