As of 2011, the philosophy of package building is to build packages based on the earliest supported release of each branch. e.g.: if on RELENG-8, the following releases are supported: 8.1, 8.2, 8.3; then packages-8-stable should be built from 8.1.
As releases go End-Of-Life (see chart), a full (not incremental!) package build should be done and uploaded.
The procedure is as follows:
Edit /var/portbuild/conf/server.conf with the following changes:
Change the value of SRC_BRANCH_branch_SUBDIR to releng/branch.N where N is the newest 'oldest' release for that branch.
Run /var/portbuild/updatesnap manually.
Run dopackages with -nobuild.
Follow the setup procedure.
Now you can run dopackages without -nobuild.