Hello dear,
.
Yes, you can do MBBS  after completing BDS.
But again you need to clear entrance exam that is NEET.
Once you qualify then your admission will be done in college according to your percentage.
.
You can even quit the BDS and apply for MBBS.
But you need to pay whole fees for getting your originals back.
.
Completing BDS and going for MBBS is a long process.Choose accordingly and think twice before decision.
.
Best of luck