There should not be a problem with upgrading directly to 6.3 .. For more info follow this document :
Upgrade path:
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_upgrade.pdf
And follow this document on upgrading:
http://www.juniper.net/techpubs/software/screenos/screenos6.2.0/upgrade_guide.pdf
Upgrade path:
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_upgrade.pdf
And follow this document on upgrading:
http://www.juniper.net/techpubs/software/screenos/screenos6.2.0/upgrade_guide.pdf