A family tree of this type is grafted together so the individual parts cannot revert as such. That said, a family tree can become unbalanced. That is one or even two parts can fail to thrive while the other grows bigger and bigger, so in time a single type rather dominates.
Study your tree carefully. You should be able to see the original grafting points where the top scions were joined directly to the rootstock, or possibly to one another. Follow the growth upwards and count how many main branches are growing from each type. If you have at least one strong main branch of each type, you need not worry about the tree becoming unbalanced. Basically you are looking at the growth to assess if each of the three elements are still doing well. Even if it is a bit out of balance sensible pruning should even it up without much difficulty. Given it was in very good health last year it seems very unlikely to have deteriorated in a single year.
According to the books the concorde is slightly later flowering than the other two, and the conference is partially self fertile. But I think Ace is on the right track. Pears are more sensitive to frost than apples. So it is possible parts of the tree suffered with frost but parts were a bit luckier and missed the damage. The way frost hits can be astonishingly patchy. I have often seen frost damage on my potatoes in an inexplicable pattern. Was the fruit mainly on one part or side of the tree? If so and the tree is free of other disease, my bet would tally with that of Ace namely frost damage to the early blossom.