Well, anything's possible - it's merely a question of how much effort is involved in making the switch.
Broadly speaking, if you're talking about pumping out the zinc bromide and replacing it with a different electrolyte, I would say that it won't work. Bear in mind that my university degree is in computer science, not inorganic chemistry (or indeed, any sort of chemistry), so if there's a conflict between what I say, and what an inorganic chemist has to say, trust the inorganic chemist. But that said, I would expect the electrode design to be fairly tightly coupled with the electrolyte chemistry.
Of the possible changes, a switch to zinc iodide would probably be the easiest, since iodine is the only one of the other halogens that isn't either intensely radioactive (astatine), or extremely reactive (ask any chemist what they think about working with fluorine gas: good quality running shoes to allow getting upwind as fast as possible would probably feature highly; and chlorine isn't exactly friendly either.) But there'd still be issues to work through, at least partially stemming from iodine being a solid at room temperature (bromine is a liquid).
Switching away from zinc halide chemistry? You'd essentially be talking about a completely new design; there might be some design principles that would carry over, but not much more than that.
There are already companies developing vanadium flow batteries, as well as iron redox batteries. Polysulfide bromide batteries have been tried, though they ran into scaling issues. Uranium redox flow batteries have also been proposed as a way to make use of the large stores of depleted uranium. So there are other chemistries out there that would be suitable for use in flow batteries, at varying stages of commercial development; I would suggest that the most promising (specifically: vanadium, iron redox, and zinc bromide) are already available from various companies as commercial products, with various pros and cons stemming from both the chemistry, and how it's been implemented in a given product.
Again: I do not have any degree in any sort of chemistry; my last chemistry subject was in Year 12. If there's any disagreement between what I say, and what an actual chemist (especially a chemist who specialises in inorganic chemistry) has to say, trust the chemist over me.
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