The transition began in 1947 when plexiglass manufacturer Rohm and Haas teamed up with Shell Oil to develop the first internally illuminated plastic sign for use in gas stations. The die-cut Shell in porcelain was expensive to manufacture, so Shell jumped in as the first to experiment with plastic. That first sign was two plastic faces assembled back to back over an internal frame that supported incandescent lights. An access cover was made into the bottom, to open the sign to replace lights. The sign was an immediate success, and thousands were installed at Shell stations nationwide over the next several years. That sign was revised in 1954 (metal frame between the faces), 1960 (red controlled background) and in the 1970s with the international Shell logo.
By 1950, most all of the oil companies were experimenting with plastic signs. Crown Central and Tydol were early converts, but others that began using signs in limited numbers were Amoco, Ashland Conoco, Derby, Esso and affiliates, Gulf, Marathon, Pure, Richfield (NY and CAL), Shamrock, Skelly, Standard (IN), and Sunoco. Unsuccessful experiments with sign, mostly due to poor lighting appearance, led other companies such as DX, Sinclair, Phillips and Mobilgas to revise their logos to make use of the signs. All of those brands introduced a new image between 1955 and 1960. Some companies lagged behind. Atlantic had very few plastic signs before their reimage in 1966. Chevron used porcelain with neon in their western region until 1970. Cities Service revised their logo in the late 1950s to use plastic. Sohio did little with plastic until about 1960. Every oil company was different. All during this time, every company had an alternative to an illuminated plastic sign, something in porcelain or occasionally tin, to use in locations that did not justify the investment in a light up sign. As for Citgo, there were a very few porcelain minor (small 4' x 4' appx) signs made for the 1965 reimage. I've never seen a full size example. Atlantic went big for plastic in 1966, and very few of the 1966 logo porcelain signs were made. Chevron switched to plastic with the 1970 reimage (they had used plastic since the Calso rebrand in the northeastern US) and I've never seen a new logo Chevron sign in porcelain from the U.S. There were thousands of porcelain Exxon signs made for the 1972-73 Exxon introduction, simply because Esso had so many small accounts where porcelain Esso signs were being replaced. Some porcelain Union signs were used in the east for the 1970 Pure rebrand, but very few. The 1974 reimage that came with the reintroduction of the Amoco brand (and the introduction in many parts of the country) was the last major oil reimage that included porcelain signs on any serious scale (not counting an occasional historic district sign or something along those lines). Porcelain signs were not necessarily removed where in place, just not created for new images. In 1990 when BP replaced the Gulf image in the south, numerous lithographed aluminum signs were made for that rebrand. They were not porcelain (I have owned one) but were made for use where a porcelain Gulf sign was being removed. One interesting side note, in the Ghent section of Norfolk Virginia, a Texaco banjo pole with a red circle/white star (1980 logo) sign existed. I'm pretty sure it was painted aluminum.
The introduction of plastic signs has been the subject of several articles in Petroleum Collectibles Monthly, and signage in transition is pointed out throughout my book, One Hundred Years of Gas Stations.