Seamless upgrade and single copy template should not be used together. At least, use this combination with caution.
If you enable the “Single Copy Template” option for your old template (that you used with previous version of Notes), seamless upgrade will fail, and database may become corrupted.
Seamless upgrade perfectly works with normal termplates. It may work if your old template is normal, and new termplate is single copy; however, I did not tested this combination.
HTH,
Alex