Unfortunately no, you can't replace an element of the periodic table because the ordering of the elements on the periodic table are based on the number protons found in the nucleus of the atom. However, you may have another option.
Carbon has 6 protons, always. Give it an extra neutron? Still carbon. Give it an extra electron? Still carbon. Give it an extra proton? Now it's nitrogen. No getting around around it, any atom with 6 protons is defined as carbon, so you can't just replace that spot number six with a different element.
However, that doesn't mean that all carbon acts the same. Diamonds are made of carbon. Graphite is made of carbon. Why are they so different? Because of the atomic structures that carbon can stably form. Allotropy is the ability for some chemical elements(like carbon) to take multiple forms, and this can result in vastly different properties from the same base element.
So, while you can't scientifically replace an existing element with a new element, you certainly can semi-scientifically say that some element has weird properties when structured a certain way that nobody had yet tested, and that simply nobody had tested that form yet in our world.