QB Issue Resolution:
You might like to consider creating a new pay rate for this employee so in the pay run which the change take place, you can seperate the number of days/hours were paid with the former pay rate and the days/hours were paid with the new pay rate as 2 seperate earning lines.
I found these 2 articles from our Payroll support for your reference:
Updating Employee Pay Rates
Adding Earnings Lines To A Pay Run
Resolution for Issue 'How do I change an employee from casual to full time, with increased pay rate, mid pay period?' available: Yes (Solved).
Source: Intuit Community forum.
How do I change an employee from casual to full time, with increased pay rate, mid pay period?: 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 Employee
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Error Recovery for Employee [Canada]
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