Varieties within a species can usually be grafted or budded interchangeably. For example, Pink dogwood can be budded or grafted onto White Flowering dogwood rootstock, pecan to hickory, a new pear ...

What I think has happened is that your pear was grafted onto the rootstock of a quince, and the quince has now taken over the tree. Grafting is common in fruit trees, roses and other plants.