taxonomy - On which date did the official name change of Lactobacillus sanfranciscensis change?

Lactobacillus sanfranciscensis first shows up in pubmed in Gänzle et al. (1998). They reference Trüper and De'Clari (1997) for the name Lactobacillus sanfranciscensis. The latter say:

As none of them makes sense in the nominative apposition construction,
we hereby correct these names to forms that are in agreement with Rule
12c as follows.

...

Lactobacillus sanfrancisco ("the city San Francisco") is corrected to Lactobacillus sanfranciscensis (adjective: "from San Francisco").

That issue of Int J Syst Bacteriol was published 1 July 1997, so I guess that's your date.

References

Gänzle M G, M Ehmann, and W P Hammes. 1998. Modeling of growth of Lactobacillus sanfranciscensis and Candida milleri in response to process parameters of sourdough fermentation. Appl Environ Microbiol 64(7):2616–2623.

Trüper H G, De'Clari L. 1997. Taxonomic note: necessary correction of specific epithets formed as substantives (nouns) "in apposition." Int J Syst Bacteriol 47:908–909.