Whether to issue a permanent or temporary UPN to a student enrolled as a Guest student at a Nursery is a decision that the education setting have to decide for themselves.
If the child has a permanent UPN it will need to be used throughout their education, in all statutory situations (e.g. school census and assessments).
You have the option to issue a Temporary UPN and any future school the child attends in can then allocate a Permanent UPN. You are best advised to seek advice from your Local Education Authority.
Please refer to this Department for Education guide for further information: Unique Pupil Numbers (UPNS)