This doesn't sound like the right tool for the job in the first place. Couldn't you just
insert into new_table select * from old_table, if they are in the same database?
But given that you already have the dump file, the most straightforward way might be to load to a scratch database, rename the table, and retake the dump from there, then drop the scratch database.
Alternatively, you could use "pg_restore" without a "-d" option to dump the custom format file to a plain text dump file, then edit that file manually to change the name in each location it occurs. Editing a file with 15 millions rows can be quite a chore, but it should be possible with something like "vi", or perhaps "sed" (but there you have to be careful not to change more than you intended to).