You can switch from chlorine to bromine at any time. It's going the other way around from bromine to chlorine that requires a water change.
You don't need to buy anything new to test for bromine. You can use your existing chlorine test kit. Just multiply your FC reading by 2.25 to get ppm Total Bromine. If you use the R-0003 and measure the CC separately, then add it to your FC reading -- for convenience you can just start off the test adding the R-0003 drops to get Total Bromine immediately. Technically, you shouldn't see CC in a bromine spa since bromamine (or monobromammonium ion) is supposed to register as FC as it reacts with the DPD dye, but some have reported needing to add the R-0003 to see all the bromine.
You don't need to worry about CYA with bromine as they do not bind to each other. Just target a 4-6 ppm bromine and you are good to go. If you have an ozonator, that adding sodium bromide to create a bromide bank should have the ozone create bromine for you so you might not even need to use bromine tabs in that case.
You don't need to buy anything new to test for bromine. You can use your existing chlorine test kit. Just multiply your FC reading by 2.25 to get ppm Total Bromine. If you use the R-0003 and measure the CC separately, then add it to your FC reading -- for convenience you can just start off the test adding the R-0003 drops to get Total Bromine immediately. Technically, you shouldn't see CC in a bromine spa since bromamine (or monobromammonium ion) is supposed to register as FC as it reacts with the DPD dye, but some have reported needing to add the R-0003 to see all the bromine.
You don't need to worry about CYA with bromine as they do not bind to each other. Just target a 4-6 ppm bromine and you are good to go. If you have an ozonator, that adding sodium bromide to create a bromide bank should have the ozone create bromine for you so you might not even need to use bromine tabs in that case.