Yes, you can change a part-time employee to full-time even if you're in the middle of a pay period, @jaf05c. I'll provide the details so you can modify their info and process your payroll in QuickBooks Online.
Since the field for full-time and part-time is currently unavailable in the program, you may utilize the Job Title area in the Employment details section of the employee's profile to enter their new status.

As their standing has changed, you'll want to update the hourly rate to the regular one. Because you're still in the middle of the payment term, you need to include the old rate with the latest to get an accurate calculation of their payment. These are the steps to follow:
- Go to the Payroll menu and select Employees.
- Choose an employee, and in the employee's profile, scroll down until you see the Pay Types section, then click Edit.
- Enter the new full-time charge in the Rate per hour field. You could also change the Hours per day to match the regular time.
- Hit the + Add hourly pay and type in the Part-time Hourly Pay name and the old rate.
- Once done, press Save.

When you generate the paycheck for this period, manually enter the exact hours worked corresponding to the status to get a correct gross pay. It would look like this:

Afterward, remove the additional part-time hourly pay so it won't affect your next payroll.
In addition, there are different payroll reports accessible within QuickBooks. These will help you view useful information about your business and employees.
By this, you'll be able to manage your employee and their payroll. I always have your back, so feel free to enter your question in the comment section. I'll be glad to help you out. Have a great day!