Yes, if you do not attend any classes in your first year of BDS and decide to leave the course to join MBBS as many candidates do it earlier too just because they don't get MBBS they choose BDS, you are likely to face difficulty in getting your submitted documents back from the university, as most institutions have strict policies regarding withdrawal from a course, especially when it involves non-attendance and potential academic misconduct.

Visit your university for transfer process they might help you in this you may have to pay bond amount for leaving the college in between.