The honest answer is no. Here is why a mid-cycle switch causes weeks of trouble, and the small refill that does the same job in 24 hours.
No, you cannot cleanly switch from chlorine to bromine without draining. It is technically possible to start dosing bromine into a chlorinated tub, but the two chemistries interact, the test strips become unreliable, and you end up resetting everything anyway. The reliable route is a partial drain to a quarter full, a fresh top-up, then starting bromine and adjusting to 4 to 6 ppm, which does the job in about 24 hours.
Free chlorine and free bromine are measured by different reactions. A test strip designed for one will give nonsense for the other, and combined strips are forgiving rather than accurate.
More importantly, the cyanuric acid (stabiliser) you have built up under a chlorine regime does nothing for bromine. It just sits there raising your TDS and your alkalinity demand. If you are weighing up the two sanitisers before committing, our chlorine versus bromine comparison covers the trade-offs.
Drain to one quarter full. Top up with fresh water. That single dilution drops the residual chlorine and stabiliser to a level the new bromine will not interact with badly.
Set up the bromine feeder or a starter dose of sodium bromide as the bottle directs. Bromide in water needs an oxidiser (chlorine or non-chlorine shock) to convert to active bromine, so add a one-time small chlorine dose to kick the cycle off.
Let it run for 12 hours with the jets on the longest filter cycle. Test free bromine in the morning and adjust to 4 to 6 ppm.
The water will feel slightly different. Bromine has a softer feel and a less sharp smell.
You may need to dose slightly more often initially while the bromide reservoir in the water builds up to a stable level, and fast-dissolving bromine granules are useful for those quick top-ups.
After a week, you should be on a normal bromine routine: a feeder topped up monthly with bromine tablets, weekly tests, and a non-chlorine shock every fortnight.
A one-off chlorine top-up in a bromine tub is fine. The chlorine just oxidises and the bromide reservoir continues to do its work. Do not make a habit of it.
BCDMH (bromochlorodimethylhydantoin) tablets contain both chlorine and bromine. They are used in some commercial tubs and avoid the choice altogether, but they need a feeder rated for them.