I opened an older world of mine (1.7.9) because my house was set up on the edge of a Deep Ocean Biome and I had been told that Ocean Monuments would spawn in older worlds. When I got in, I immediately found Guardians right next to my house, but there was no Ocean Monument anywhere. Only Guardians spawned, not Elder Guardians.
What should have happened: Guardians should have spawned around an Ocean Monument, or not at all.
What did happen: No Ocean Monument, but Guardians being their normal annoying selves all the same.
Although the Wiki says, "Ocean Monuments can spawn in existing chunks from older versions of Minecraft," it appears they do not, and only the Guardians spawn.
In my current world save, no monument spawned in an area I had previously explored, but I did find Guardians. So I loaded an older backup, in which that same area had not been previously explored, and found a monument.
So it appears that monuments will NOT spawn in existing chunks, but the Guardians still do.
This issue is NOT resolved!