When you switch your subscription plan, all your previously acquired asset licenses are converted to your current license. The Terms of the licenses can be found over at quixel.com/terms.
When you cancel a subscription, the Terms of the last active license will apply to any acquired/downloaded assets.
When you switch between any paid Quixel subscriptions, you keep all your previous acquire history.
If you switch to the Unreal Unlimited plan, any future use of assets that you acquired under the same account but on a previous plan will also fall under the same license and will be treated as UE-Only Content as described in the Unreal Engine End User License Agreement.
Please note that assets purchased with each plan have different license restrictions and can not be used interchangeably. If you choose to have two accounts, we strongly recommend that you define different download and storage destinations for each account. Any assets that you had on your local drives while under the Unreal Unlimited plan will remain licensed as UE-Only assets.