Yes it does go away when you could have normally collected SS, BUT you would have been eligible to work then any way... I'm looking at the first 2 years for me - age 64-66... and I'd probably get near $0 anyway if I worked or < 50% of it. Thus all it would do is cut my SS income in future years. It sure would help now though... about $1800 / mo !!
IF I hadn't been contacted by 2 different Dept chairs about teaching- I'd probably go for early SS, but looks like I'll have some significant outside income until I'm 66. I may
have a chance to go back full time next fall or spring 2014 !! I don't know if I want to do that, but I could probably make $55- to $60,000 doing almost what I'd do part-time (at one third the salary).
When I was at CDC and teaching part-time at GPC, for years I taught eight sections of microbiology: three in the spring, two in the summer, and three in the fall. For those services, I got paid about $17,000 ($2200 per section). A full-time faculty at GPC has to teach nine sections per year and makes 2.5 to 3 times the salary.
Of course I couldn't give up CDC, because I was making a lot lot more there.