Mature Cherry Tree pruning

I have two mature Cherry trees that I need to prune. The property is about 90 years old and I have been told that most of the fruit trees are from the original farm that the houses were then built on so I believe the trees may be this sort of age. The trees still flower but the Cherries have minimal flesh around the stone and are very bitter. I have read articles that say you should prune Sweet Cherries in the summer and Bitter / Sour Cherries in the winter but then other articles say all should be pruned in the summer. I have also read articles that say you should prune hard to let light and ventilation in but other articles that say you should graduate the pruning i.e. take 2-3 feet back a year until you get closer to the desired size. The trees have not been pruned for at least 8 years and most of the growth is at the end of long thinner branches so I am also worried that if I prune hard back to the old large branches that re-growth wont occur and the trees may die. Any advice will be welcome.