The short answer is that the change from upper to lower case signified a re-branding of the trademark Pyrex® in the late 1970s but is not a conclusive way to determine, historically, what type of glass formulation the product is made from.
Rather than being a type of glass, Pyrex® is a brand that represents multiple types of glasses and has been manufactured by several different companies. The brand was first established in 1915 by Corning Glass Works (eventually Corning, Inc.) In 1998, Corning, Inc. sold its consumer products division to Borden (later World Kitchen) and licensed that company to use the Pyrex®name. Currently, Corning, Inc. licenses the company Instant Brands to produce Pyrex branded dishes in the United States. Corning licenses the trademark to other companies as well, including International Cookware, based in France, where it is made from tempered borosilicate glass.
According to research done by Pyrex®collector Dianne Williams, over 150 different glass compositions have been used for Pyrex branded products since 1915, including different formulations of borosilicate, aluminosilicate, and heat- and air- tempered soda-lime glass.
At some point, Corning, Inc. began to experiment with creating transparent ovenware with tempered soda-lime glass. Likely this change began when Corning started producing its popular opalware in the 1940s, which is made from a soda-lime formulation.
Some manufacturing plants switched to tempered soda-lime formulations, while others continued to produce borosilicate Pyrex® products. According to Herb Dann, a designer at Corning, Inc. from 1961 until the 1990s, by the time World Kitchen acquired the license to produce Pyrex®, Corning had mostly switched to tempered soda lime glass for almost all of its tableware products. The exception, said Dann, in his 2014 interview with the Corning Museum of Glass, was the 13x9x2 pan, which he said Corning never manufactured with soda-lime glass. Corning's Charleroi plant was a major producer of transparent Pyrex ovenware made from air-tempered soda-lime glass (Paul Topichak interview, 2014).
According to Dave Huber, a product liability specialist at Corning, even plants that used only soda-lime formulations were using a variety of batch recipes depending on the plant. Huber also notes in his 2014 oral history, that Corning relied on a complex system of dots and dashes (known as manufacturing date codes) stamped on its products to identify what type of glass and when and where it was produced.
Most sources agree that tableware sold in the United States under the Pyrex® license is primarily tempered soda-lime today. However, according to the Instant Brand site, a current licensed maker of Pyrex®, the company does also use borosilicate for some of its products. They do not identify, however, what those products are. https://www.pyrexhome.com/support/frequently-asked-questions
The various formulations for Pyrex glass have been a source of controversy for many, as well as a source of consumer lawsuits over shattering bakeware. As a result, the Consumer Product Safety Commission investigated this question and concluded that borosilicate glass is much less likely to break under thermal shock, while tempered soda lime glass is less likely to break on impact. The New York Times explores this topic on its Wirecutter blog in the article, "Why we are not worried about Pyrex bakeware exploding."
For more about Pyrex history, including interviews with Corning employees who worked with Pyrex products, consult our Pyrex Research Guide.