When a student attends Nursery for the first time, the Nursery have the choice to allocate a permanent UPN number to the child or a Temporary. If they have only allocated a Temporary UPN, which the ScholarPack system will recognise as temporary, as the End letter will be A - please see the link below, which will explain this in more detail.
As the students are now starting in Reception, you are now able to provide the student with a permanent UPN. To do this you can go to Admin>Students>Bulk UPN's and do them all together -
Or, you can go to the Student profile, their Extended tab and under the School information area, allocate a Permanent UPN. Scroll down and save at the bottom of the page.