for ITIN and SSN number

If F-1 file W-7 for ITIN ( reason f: nonresident alien student,professor,or researcher filing US tax return.), does he/she has to file tax return form? The application instruction didn't say so. But could the application be denied?

If one gets ITIN, can he/she get a Social security number later on? It seems you cannot
get them both. How to replace ITIN number?

Thanks again!