I suspect that if the mode of virulence is the same, then previous vaccinations would be effective against even a genetically altered form of the virus (although this is far from certain).
There is also considerable question in the mind of immunolgists, as to whether or not those vaccinated against smallpox would maintain their immunity for so long without a booster. Precious little experience in this area.
The immunity is theoretically lifelong - smallpox vaccinations did not, under normal therapeutic practice, require a booster. However, typically viral immunity of that type fades with time as the proportion of antibodies in the immune system sensitive to that antigen goes down gradually. High doses of the virus, well beyond the disease threshold, may in fact overwhelm somebody whose immune system might otherwise be able to "ramp up" upon exposure - normal doses might do the same to an individual who is immune-challenged for some reason (drugs, other diseases - AIDS is only an extreme example). Hence it will differ from individual to individual based on time since inoculation, health, and degree of exposure.
I am not privy to current data on warfare-intended "hot" smallpox virii, but typically the difference is one of degree of virulence and not of mechanism. This is not necessarily true for bacterial agents, which may exchange large amounts of genetic data by themselves in vitro, much less at the hand of man - this is thought to be the means by which the otherwise harmless Escherichia coli bacterium has picked up enteropathogenic properties.
In short, if you have one of those little scars on the back of your shoulder, you're much safer than those without it, but still not necessarily safe, nor can you be vaccinated to a level that confers 100% immunity against all doses. Uh...have a nice day...