No, high risk cannot become low risk. The absence of warts does not mean you don't have low risk too. Most sexually active adults have both high and low risk strains. Multiple strains are very common. If you had high risk while dating him, then then he probably has it too but he can't be tested for it.
For some reason, people with the low risk strains don't always get warts. No one knows why but the weakness of one's immune system may cause warts to appear. 50% of all people who have the low risk strains do not get warts and since men cannot be tested for it, they think the absence of warts means that they don't have it. That's just wishful thinking....