Since Bromine contains chlorine in it, you can go from Chlorine to Bromine without changing the water.
Bromine, itself, is an element and does not contain Chlorine.
Bromine TABS, do contain chlorine. As a result, if you put Bromine tabs in a floating feeder in your spa you will have a heavy chlorine gas cloud eating away at pillows, and the inside of your cover.
If you want to use Bromine, buy it pure, and do your own shock once per week with chlorine, MPS, or whatever oxidizer you like.