Layouts and tables are not the same thing. You can have many layouts that all refer the same table.
The safest option is to leave all the records in one table. A FileMaker table can store millions of records so a few years worth of project records isn't likely to be too much for your database to handle. You can set up one layout with scripts that limit the records shown to only those where a status field equals a value that marks it as completed and a different layout can be used to display the records in the same table that are not complete.
If you do choose to copy completed records into a second table, Import records can be used to copy one or a group of records from one table to another.