Basu,
At this point, it is too late to change your housing preference form in any way. Judging from your housing card selections, you indicated one of fourteen spaces in the Wellness (substance-free) section of Watson Hall (campus map building #5:
http://www.lafayette.edu/community/campusmap/). There are only sixteen spaces on the Wellness floor of Keefe Hall, all of which will be claimed by those who indicated "Wellness" and "Co-ed by room" on their preference forms.
Assuming you will be assigned to Watson (even though you indicated it on your preference form, there are no guarantees), just know that it is a fine living environment. If you plan on majoring in either Engineering, Chemistry, Biology, or Physics, you will be within a one minute walking distance from all of your classes.
You can always live in Keefe Hall on one of the special-interest floors starting your sophomore year. I wouldn't apply to become an RA just to get into Keefe because, even if you are offered an RA position, you are not guaranteed to be assigned to Keefe by the Office of Residence Life. Only apply to become an RA if you genuinely want to become an RA.
Take care,
- Justin