When diagnosed early even the life threatening malignant melanoma can be treated and cured. Skin cancer of the foot are considered more dangerous because they usually are not diagnosed early. It is very important to understand that every mole on foot are not melanoma, and even if the mole change its color, size it can be harmless. Many factors play role like excess sun exposure, family history, age, race and so so.. if possible can u post a clear picture so we can have a look.
Yes it can happen, melanoma can arise in pre-existing moles, some atypical moles that looks harmless for years can turn into melanoma, so no one can confidently say the moles are harmless, they can turn into melanoma anytime. Being said that, ur race, sun exposure, family history, excess moles and several other factors play an important role. I know everyone worries in such situation, so a better option would be just cut it and no worries then after.