I just read Codename: Sailor V and there were many references to Sailor Moon (Usagi and the other guardians making cameos, etc.), in spite of the former being created and published first. It was especially prominent towards the end, but Usagi made an appearance pretty early on. I'm interested in knowing what the publication history of Codename: Sailor V was like. And if any edits were made, what they were and when they happened. A timeline of Sailor V and Sailor Moon might also be sufficient to answer the question, if my assumption about edits is incorrect.
Codename: Sailor V first appeared as a one-shot on August 3, 1991.
The actual manga was then written pretty much simultaneously with the Sailor Moon manga, which started only a few months later (December 28, 1991) and which was updated much more frequently.
It even ended later (July 3, 1997) than the Sailor Moon manga (February 3, 1997).
That allowed for the cameos between the series.
Just to elaborate a bit on the other answer, I found a helpful timeline at the end of Pretty Guardian Sailor Moon: Short Stories Volume 2. I'll pull out the relevant entries. This is from the Kodansha English translation, published in 2013.
August 1991 / "Codename: Sailor V" debuts!!
Naoko Takeuchi-sensei's one-shot short story, "Code-name: Sailor V," which becomes the basis for "Sailor Moon," appears in RunRun's Summer Break issue.
December 1991 / "Pretty Guardian Sailor Moon" series launches!!
The long-awaited manga serialization finally begins, in Nakayoshi's February issue.
March 1992 / "Pretty Guardian Sailor Moon" animated television series begins airing!
March 1992 / "Codename: Sailor V" appears again in RunRun's Spring Break Issue!!
"Sailor V" also becomes serialized, and appears concurrently with Nakayoshi's "Sailor Moon." The following April, RunRun gets re-launched as an every-other-month magazine.
February 1997 / "Sailor Moon"'s emotional finale!
The manga series that has spanned five years' time comes to a happy end in Nakayoshi's March issue.
November 1997 / "Codename: Sailor V" reaches a climax!
The finale of the Sailor Guardian series originator, "Sailor V," appears in RunRun's November issue.
September 2003 / "Pretty Guardian Sailor Moon Revised Edition" (12 volumes en toto, plus 2 additional volumes) begins publication!!
October 2004 / "Codename: Sailor V Revised Edition" (two volumes in total) begins publication!!
So, it seems that only the first chapter of Codename: Sailor V (the one-shot) was published before the beginning of Sailor Moon. Additionally, Codename: Sailor V was released at a slower pace and came to an end after Sailor Moon. The end of the timeline mentions revised versions, but doesn't elaborate on what sort of changes were made.