First of all you don't need to unpublish a game, i.e. If you have used a paid Unity license, say "Plus", to build and publish a game, and didn't earn at least 100K+ in the past year, you don't need to unpublish it in order to downgrade to "Personal".
However, if you downgrade to "Personal" and make any modifications to the game that was published using "Plus", the next build will be with the "Personal" license, and the corresponding update published will be a "Personal" version, i.e. with the Unity Splash Screen (no customization), as well as all other "Plus" features removed.
Please refer to Terms of Service:
Article "4.2 Your Content" (you own the content you create, hence you cannot be forced to unpublish something you own)
and
Article "5.3 Subscription Terms" (subscriptions can be cancelled after commitment period). Therefore, if you cancel, you still own the content.
and
Article "6. Compliance" (Unity or its representatives can inspect your systems at your facilities or remotely, so Unity can verify if your revenue/funding corresponds to your active license. Therefore, your active/current license must correspond to your revenue/funding in the last 12 months, in order to comply with the TOS). For more detailed description of the 12 months period see also Tier Eligibility in the Unity Software Additional Terms: "Tier Eligibility is measured for the most recent twelve (12) month period, etc."
Also please refer to the FAQ:
"Do I own the content I create with my subscription? => Yes, you fully own the content you create with a Unity subscription, also if you stop subscribing to Unity."
and
"Will my subscription automatically renew? => .... ... If you do not wish to continue on a flex plan, you can choose expire your subscription after its commitment period. If you choose to expire your plan, your version of Unity will switch to Personal.".
You can always contact Unity Sales to ask them any question directly, they are very helpful: https://unity3d.com/contact/get-in-touch