Bringing an entirely new automobile to production is expensive. There are few cases in the modern (say, from 1930) history of the industry that a platform was in production for only one year. Circumstances can mitigate, but Honda for many years tended to do a redesign of a model every four years. Some makers would redesign every two or three years and others (think Volkswagen Beetle) produced the same platform for decades.
The term "platform" is auto industry jargon for a set of core components. That platform can serve as the basis for a single model (Toyota RAV4, for example) or it can be shared by several makes or models that are differentiated by relatively superficial appearance changes (for instance, the 1990s GM-10 platform was used for Chevrolet, Pontiac, Buick and Oldsmobile models).
Nowadays a platform comes in the form of a unitized body into which are stuffed engines, drive trains and passenger accommodations and onto which are attached body shell panels. Before the 1970s, most American makers used separate bodies that where bolted onto a chassis. Unit bodies are expensive to engineer and modify for styling reasons. Separate bodies were changed less expensively, but the one area that really did cost a lot to redo was the cowl -- the part comprising the engine firewall.
Thanks to iron rules of the economics of scale, car makers with comparatively few sales could not afford to change bodies as often as their larger, richer competitors. So to keep their models as stylistically fresh as possible, they relied on facelifts to entice customers. A "facelift" in automobile terms means restyling visible parts of a car while retaining the same platform. This can be done once or twice, but eventually the car-buying public would wise up to the fact that warmed-over goods were being offered whereas other companies were selling new goods.
Below are examples of facelifts from the 1950s when annual styling changes were probably the major marketing tool.
Lets pause before dealing with the 50s and consider Hudson. Hudson brought a new body to the market for the 1936 model year and continued using it with facelift after facelift through the 1947 model year. The 1937 car show above is very similar to the '36 and can be taken as the starting point. It seems a lot different from the 1946-47 Hudson. But look carefully. Note that the windows are essentially the same. And the body tucks inward from the belt line to the running board area on both cars. What we see is the 1936 basic body with new fenders, trunk and hood (not to mention the grille, chrome trim, etc.).
Kaiser-Frazer axed the Frazer brand after the 1951 model year. For 1951, the Kaiser got a svelte new body and Frazer a massive facelift. Given the fate of Frazer, that facelift strikes me as being a huge waste of money. Nevertheless, the facelift was so effective that young punk me failed to realize at the time what it was; I thought it was a new body design.
These photos illustrate the first and final models using the big-Nash bodies of the 1950s. The big (and most expensive) change was the addition of a wrap-around windshield, a styling must at mid-decade. Otherwise, the facelift was mostly in the form of larger front wheel openings, quad headlights, revised grille and trim: not as drastic as for the other examples here.
Ford definitely was not a low-volume producer, but the decision was made to give its 1952-vintage bodies a major facelift that would extend their production two more years. Changes included the competitively necessary wraparound windshield, a slightly lowered roof (it was flattened), revised fenders, "Frenched" headlights and a new grille. These changes effectively created the image of a total redesign.
Mercury also had its 1952 body facelifted for 1955. The types of changes made were similar to those for Ford and, again, the result was the appearance of a total redesign.
Packards received their first post-World War 2 total restyling for the 1951 model year; these bodies continued in production through 1956. These photos show top-of-the-line Packards for those years. As with Ford and Mercury, 1955 was the year for the major facelift. The expected wraparound windshield was added, the fronts of the rear fenders were squared off somewhat, the front end was restyled as was the rear. Chrome strips, aluminum panels and paint two-toning helped give the sides a different look. As with Ford and Mercury, to a casual onlooker the new Packard seemed to be all-new.
These photos show how entry-level Packards were facelifted into the the short-lived Clipper brand. Changes were made in line with those for senior Packards, though details varied. Grille bars were vertical rather than a grid, the side strips and paint differed and the tail lights and rear fender tips were made less sedate.
Studebaker introduced its last truly new bodies for 1953. And, like Hudson, several major facelifts were undertaken. The 1964 Studebaker looks totally different from the '53 even though the "bones" are nearly the same. The '64 shown is a hardtop coupe; I couldn't fine a suitable photo of a sedan which might have indicated similarities better.