You don't say where you live. Whether the licence becomes 'void' depends on the regulator. The FAA one doesn't 'void' but is dependent on a current medical.
Essentially an ATP lets you be PIC on a scheduled service like what Virgin or United provide. Part 121 in the States. You don't need an ATP to be first officer, though in the States you generally will not get a job in a major without one. That's because you will usually need to have first worked in a regional where you would be captain of a scheduled service, just in a smaller plane.
You can fly PIC charter with a CPL, but need a certain amount of hours. Fresh CPL with 250 hours (or whatever) only lets you instruct, fly banners etc etc. Charter companies tend to hire people they think they will get on with.