QB Issue Resolution:

no, you would have to delete the expense entry and enter the bill

BUT, if you have paid the expense there is no reason to enter a bill and then pay it.  enter bills is just used for when you get a bill but will pay it later.

Resolution for Issue 'how to change an expense to a bill' available: Yes (Solved).
Source: Intuit Community forum.

how to change an expense to a bill: 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 BILLS
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