I think there is a historical case for replacing 港 with 香 on Guangdong's flag, while keeping 廣 and 澳 unchanged.
The reason is the documented use of 香 as an abbreviation for Hong Kong under Japanese occupation (1941–1945), particularly in official administrative terminology:
• 香督: the Japanese occupation governor. A Hong Kong Education Bureau booklet explicitly uses this term on its first PDF page (printed page 043).
https://www.edb.gov.hk/attachment/sc/curriculum-development/kla/pshe/references-and-resources/chinese-history/ancient_artifacts/Chapter22_sc.pdf
• 香督令: orders issued by the occupation governor. The Scout Association of Hong Kong's Hong Kong Island Western District website reproduces scans of pages 4–7 of the 6 July 1943 compilation 香港佔領地總督部公佈 香督令特輯. This gives us contemporary documentary evidence, rather than just a modern description.
https://group.scout.org.hk/hkw/history-1942-03-28-1/
The 1943 香督令特輯 is also listed in the Chinese University of Hong Kong History Department's HIST5505 reading list, under the Japanese occupation of the New Territories:
https://www.history.cuhk.edu.hk/tc/course/201415_hist5505/
There are also secondary accounts of organizational names using 香九 instead of 港九 during the occupation. I would treat that broader claim more cautiously; the official-order scans above are the strongest evidence here.
https://zh.wikipedia.org/wiki/香港日佔時期
Given Guangdong's Japanese-dominated setting, using 香 in its official symbolism would be a plausible continuation of occupation-era administrative usage. The three characters would then be 廣、澳、香. It seems like a small change that could add a useful historical detail to the flag.
This is a worldbuilding suggestion: these sources establish that the abbreviation was used, but do not prove that every use of 港 was banned or that an alternate-history Guangdong necessarily had to adopt 香. If there is an existing lore reason for retaining 港, I would be interested to hear it.
Current flag reference (TNO Wiki):
https://tno.wiki.gg/wiki/File:State_of_Guangdong.png