It's generally best to change one thing at a time, so we can tell what action we took caused the change that follows (assuming it didn't happen by coincindence, which is always a risk). At some point, though, your experimental question may become "can I stay well off antibiotics even if I do *everything* else that makes sense." That's a very different question than "can I stay well with one antibiotic instead of two?" To answer the first question you might have to do several things at once (e.g. significantly cut carbs and add VSL #3 DS), since otherwise you might start feeling poorly and still not know the answer. In any case, you may have more than one question about what might work, and it's likely that you're best off tackling one question at a time, even if you must change several things for some of them.
For what it's worth, I tried this experiment myself about eight months ago or so, in consultation with my GI. I was already using VSL, but I drastically cut carbs when I stopped the antibiotics. Unfortunately it didn't take long to get my answer, which is why I'm now on Cipro and Flagyl (and, for the record, doing just fine).