Fruit looks like cherries but tastes like plums!

What’s happened? We have a small orchard with old fruit trees (in Cheshire) including plums and cherries (plus apples etc). Many trees, including some that have always had plums on them before and some that have always had cherries on are now full of ripe fruit that looks exactly like largish cherries (colour, shape, skin) but they have the texture and taste of plums.