Just rewrite the network statement of Area 1.
According with what you decsribed nothing else should be impacted. Of course do it during out of office hours!!
router ospf 1
network 1.1.1.0 0.0.0.3 area 1
!type instead
network 1.1.1.0 0.0.0.3 area 0
If you have any particular redistribution, take a "snapshot" of what the configuration , adjacencies , interfaces status are before and after the migration and verify the differences.
If the ISPs route are simple static routes you should not have any issue.
Alessio