QB Issue Resolution:

Sometimes you have to wait 24 hours to change plans again but yes you can downgrade back down to SimpleStart,

https://community.intuit.com/articles/1609737-downgrade-your-quickbooks-online-subscription-united-s…

Resolution for Issue 'I changed my plan to QuickBooks Online Essentials. Could I returned to the last plan – Simple?' available: Yes (Solved).
Source: Intuit Community forum.

I changed my plan to QuickBooks Online Essentials. Could I returned to the last plan – Simple?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for onlinesubscription
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Error Recovery for onlinesubscription [Canada]
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