Hello,
Although this idea seems good to opt as you'd still have a bds seat with you.
But its better to focus completely on neet for preparation rather than joining an bds college.
As you will still have to make projects, study for bds exmainations , college events etc. A drop from neet preparation is an better option as 60% of medical stidents are droppers.

However, no additional fees has to be payed except the bds 1st year fee if you leave your seat next year for a mbbs seat.
Hope this helps