Hello

Yes, it creates a problem if you're 12th LC(state OBC) and JEE(Central OBC/EWS) categories differ.

JoSAA requires a central OBC-NCL certificate for OBC reservation; since Karnataka OBC isn't central, you will be treated as general, or you can use a Declaration for OBC-to-General conversions from during counseling, but switching to EWS needs you to have applied as EWS initially. Your EWS certificate works if you meet the income criteria, but yes the important  thing is Central OBC list for OBC, not state list.

Hope it helps you, in case of any doubts you can directly drop your query or you can visit to Careers360.com