You can downgrade to any UA card - even those without a fee - but the real issue is if you can re-upgrade to it later - that may not be possible
You can not change to another line like Hyatt or Marriott or Sapphire etc etc

I closed the club card and just called for the fee rebate on the Plus- refused !
Chase does not want us any more

Thank you for contacting Chase. We appreciate your loyalty. The annual membership fee of $450.00 is charged during the same month each year and assures you of many benefits and privileges associated with your card. An option to lower the annual fee on your card ending in 8683 or a rebate (waiver) is unavailable. However, you have an option to change your current card to a low annual fee card. We're happy to help you with this change; however, we are unable to assist you with processing a product change by email because it requires a detailed review of your account and additional questions to be answered. Please call us at the number on the back of your card to get started. Further, you can redeem your miles to offset the annual membership fee within 90 days of when the fee was posted to your credit card account. Visit the Cardmember Redemption website at united.com/choices to redeem miles for a statement credit in the amount of your annual fee. We appreciate your business and thank you for being a Chase customer.