I am embarrassed to have to ask this but I need clarification.
I know that buds can become either a vegetative bud or a fruit bud in its early stage of development depending on whether the tree has the right nutrients, an adequate amount of sun or not, or even whether the tree is properly pruned or overly pruned.
But what actually happens to a flower bud that has already developed and set fruit on a spur on, say, an apple tree, when the tree is, say, overly pruned? Can that bud actually revert to a vegetative state for a period of time until the conditions change (like for the tree to "settle" down and for it to then become a fruit bud? Can it be made to change permanently to a vegetative state?
The whole thing is a little confusing to me.....any clarification appreciated.

Generally speaking what kind of happens is flower buds are stimulated to grow from wood that has already formed in the summer so that by the end of summer they will always stay or die early. Die early I think I'm referring to exposure to very cold temperatures which by comparison to leaf buds -are much more survivable to the cold. This is probably why in the cold hardy fruiting species such as apples, apples form leaf buds next to flower buds always. Then over the course of a cold harsh winter if an apple tree has lost all the flower buds then by spring the leaf buds spring into life instead of an apple.

Your are right on the mark about apples needing the right conditions to bloom. There was an gardeners conference in Alaska about a year ago and during a question and answer period somebody asked why apples growing in his greenhouse never bloomed inside the greenhouse. The trees were eventually growing branches out the vents which did have blooms on them. At that point everybody debated that there were no blooms inside the greenhouse because of temperature related issues such as chilling hours etc. Then a smart person that had a PHD in botany asked what the walls of the greenhouse were made of. "Fiberglass"! Then PHD Bob says fiberglass blocks most of the proper rays of the sun that are necessary for the initiation of flowers.

little lesson: Fiberglass is obsolete and the newer best plastics "polycarbonate"s are in. Modern day window glass the way they manufacture them these days also make them unsuitable for greenhouses.

My good fellow, what makes glass unsuitable for greenhouses? I use old shower doors made of glass as a buffer tween the sun and my spring transplants!

I use my old sliding glass door to buffer tender plants in the spring as well. Old glass is great for greenhouses and polycarbonate is as well for letting all the light in. How ever using glass manufactured "these" days cause blocking of critical shades of light important to plants.

I know a little about greenhouse growing, having worked in a greenhouse complex for the last 18 years.

First off, old fashioned glass CAN be a good greenhouse covering. BUT (and it's a big but) modern window glass is more often than not manufactured as Low-E glass. Regular off the shelf Low-E glass prevents the transmission of solar heat from entering. IF you are looking to use Low-E glass in a greenhouse make sure the Solar Heat Gain Coefficient (SHGC) is at least 0.52 or greater.

I made the mistake of installing low E windows in my home and my southern exposure rooms no longer are passively heated. Here in Colorado, where our days during winter might be chilly outside, are still bright and sunny and a southern window with a higher SHGC can save us alot of the expense of natural gas heating. Had I to do it all over again, i would have put in custom low-e glass with higher SHGC in my southern exposure. the other exposures are fine and do save some energy.

A little trick to see if a used window in a recycled building materials center is suitable for greenhouse use is to face the window towards the sun and place your hand behind it. If you can feel the solar heat radiating through it and warming your hand, then it is suitable for greenhouse use. Chances are good that used shower doors will be high SHGC as low-E shower doors just don't make economic sense