There would be no logical reason for the bacteria type to go back to the original after a longterm change of diet unless you went back to your original diet - which is a huge possibility since we know firsthand that most people have trouble sticking to a diet change longterm unless they move location and lose access to their old habits. The old bacteria has little to feed on; it would not be able to thrive under the new conditions, hence why the gut type changed in the first place. The new bacteria that has taken over will absolutely continue to thrive because it has a lot of things to feed on.

It's like a yeast infection. Once the yeast has the foods it needs to flourish, it's off to the races and will overwhelm all the other bacteria that are trying to thrive. The only way to get rid of it is to completely deny it the things it's feeding on and/or assault it with things that actively kill it.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3368382/ - Study that only longterm diet changes are associated with gut bacteria changes. Short-term changes did not see it, which is unsurprising; some gentler meds can't even get rid of a yeast infection in 10 days, why would a change in diet change your gut balance in just 10 days?