Other than this point, better check with StarHub and eight as well whether there are any restrictions.
I tend to believe no restrictions but there are some strange things from StarHub.
1) StarHub new SIM Only plans cannot port in from or port out to StarHub Prepaid plans (postpaid as per IMDA classification) as per the reports.
2) eight also does not accept the port-in from StarHub Prepaid plans.
3) But then StarHub Prepaid plans have no issues to port out to SingTel/M1 MVNOs.