As per the brochure "If a candidate refuses to take admission in the allotted branch at the time of admission, he / she will not be considered in next phase of counselling except spot round." so I think you can reject your allotment and appear for spot round. However its riskey if all seats are filled by then in branch you want to take admission.
Admitted candidates will be considered for up gradation of branches. A candidate, who has been admitted to a Branch other than his/her first choice, will automatically be upgraded, as per his/her merit in JEE(Main)-2015, to a Branch of his/her higher preference as indicated in his/her online application submission.
So if you have filled CS branch as higher priority you will be upgraded to it. You need not to leave your seat.