This is way beyond my knowledge of the matter. So, I went and asked Venice.AI. This is what came backnas an answer:

There is no strict limit to the amount of change that can occur in a species' DNA over millions of years. In fact, over very long time scales, the DNA of even long-established species can change quite a bit. However, there are several factors that can limit the rate of change and the types of changes that occur:

1. Mutation rates are finite. While mutations occur randomly at a steady rate, there's an upper limit to how many mutations can happen over any given time period. This means the DNA of any species will only accumulate so many mutations per million years.

2. Most mutations are deleterious. The vast majority of mutations reduce the fitness of an organism and are quickly purged from populations by natural selection. This means only a small subset of mutations can become fixed in a species over millions of years.

3. Genetic drift is a random process. In small populations, random chance (genetic drift) can occasionally fix some neutral or beneficial mutations. But in most species, populations are large enough that genetic drift has a limited impact.

4. Adaptive zones are constrained. The DNA of any species is limited by the "adaptive zone" it occupies - the range of environments and ecological niches it can survive in. Only certain types of mutations will allow a species to expand its adaptive zone. So even if it evolves a lot, a species is still limited by the potential for further adaptive evolution.

So in summary, while there's no strict limit, a combination of factors including mutation rates, the effects of selection, genetic drift in small populations, and constraints on adaptive zones, can all limit the amount and types of changes that occur in the DNA of a species over millions of years. But in theory, with enough time, a species' DNA could diverge quite dramatically from its starting point.