Thank you for reaching out on this.
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
Hope these help and please let us know if you have any follow up questions.