Anthony, whilst I cannot of course tell you why this specific cleaner came to have an older motor fitted to it (and I know you were not expecting me to), I can forward some general information from my own long career spent repairing cleaners as a private business.
As you might expect, I saw all manner of cleaners in the 25 years or so I had the shop, and whilst a lot which required repair had never been touched with so much as a screwdriver before, a good deal more came in showing all sorts of evidence of repair, and indeed repairs attempted at various levels on the quality scale.
Hoover were wonderful cleaners to work on, as so many parts on these machines could be replaced piecemeal. This meant it was often cheap and very easy to get a Hoover cleaner up & running for a customer, especially if one had a back-stock of older cleaners, which I did. This was, of course, to the detriment of Hoover as a brand, and it never ceased to amaze me just how much assembly went into a Hoover cleaner. Not only would it have made the cleaner expensive to build on the production line, the fact it could be repaired so easily meant that sales of new cleaners could well be hampered.
I think the real turning point came with the Turbopower range, as these cleaners were designed to wear out much more quickly that their predecessors, and not only that, we are talking major parts of the cleaner, such as the chassis and main body. However, even these models had to be modified to stop them wearing out too early, such as the appearance of a washer behind the read wheel axle, as without it the chassis wore out fast. But then on the opposite side of the story, Hoover soon decided that only three clamps were needed to hold the fan cover to the motor, even though there had originally been four, and indeed the moulding was never altered to reflect this, it was just that on the assembly line only three clamps were attached. Also, the front wheels were redesigned so as to no longer need the two metal clamps to hold the axle in place.
At the same time, the U1036 junior became the U1104, and this -as I am sure we all know- was quite a departure from the previous models, as it had much more plastic in it and a whole lot less parts, thus assembly time was drastically reduced. So where am I going with this? Well, the Constellation was essentially a bullet-proof cleaner, and like a good deal of models before and after it, it was easy to take it apart and repair it. I never did find out why Hoover changed from the all-metal motor to the metal & plastic, however, I think it is safe to assume that those later motors with the hard-plastic casing would have been cheaper to make, would have been a little lighter in weight, and what I did notice for myself was that I saw more burnt-out Constellations with that type of motor than I did the older metal style. Therefore it was very much in Hoovers interest to use that type of motor, as it was more cost effective and possibly less reliable, meaning that -as I mentioned before- a sale of a new cleaner was potentially more likely.
But even so, for many years a good deal of people still sought a final professional verdict on their "dead" cleaner, from the likes of myself and so on. Now, tell a customer that the motor has failed in their cleaner, and you shoot yourself right in the foot, because as far as they are concerned, that's it, end of the story. Very few people would ever spend the money required to replace a motor, and quite honestly I could not blame them, not when it was often two-thirds of the cost of a whole new cleaner and when all the other wear & tear on their existing cleaner was taken into account. So, it was down to me to tell a few white-lies, if I may call them that, if I knew I had sufficient parts from another cleaner to get theirs working again. The customer did not need to know the whole story, the fact they could get their old cleaner back in full working order for the price of a standard service would be all that mattered.
Once, I even gave a customer an entirely different cleaner back. I'd taken a few pounds off a reconditioned cleaner in exchange for a customers old Electrolux 65. It worked perfectly, but as far as it's owner was concerned, was well past its best. That cleaner sat in my stock room for a number of years, until one day someone brought in a rather tatty model 65, with missing handle and burnt-out motor. I never told them the motor had failed, even though the customer suspected it had, and suggested they leave the cleaner with me. I then swapped the end caps and mains lead off their cleaner for the same on the one in my stockroom. When the "repair" had been completed, I telephoned the customer to say it was simply a case of a small adjustment to the motor and that I'd even managed to replace the handle. It still looked very much like their cleaner, and were delighted to get it back for a very small fee, given that they were not at all well off. Now, on the one hand it is arguably wrong to lie in business, on the other, I would never had been paid a penny for looking at that cleaner, would have been left to dispose of it as likely as not, and the customer either having to go into debt for a new machine or indeed go without a cleaner at all. So for me, it was win-win all round, and each customer & cleaner was taken on their own merit.
I expect your Hoover Constellation came to the hands of someone equally as capable as myself, although I am still uncertain as to whether or not that motor is a good fit (I just cannot remember) and indeed the fitting of the switch upside down is a little lackadaisical in my opinion, though I admit they could be awkward to align and the installer may not have wished to turn it around once the mistake had been realised.
