I agree, in all likelihood, Rathbone's Skeleton curse replaced his Shores of Gold curse. However, I was curious to understand why that was the case.
And I think I have found a theory to explain it.
This theory revolves around how, at least as far as I understand, curses appear, as well as the different "potencies" of curses operate, in the Sea of Thieves.
In the Sea of Thieves, there are two ways that a person can possess a curse. The first is by stealing treasure from its resting place, or touching treasure that has been stolen from its resting place, and this treasure has had a curse placed upon it by the Ancients. The second, according to Nine Cat Nura, “starts” “when someone feels something so strong that it can’t be undone”. These strong feelings, such as hate, rage, melancholy, etc. lead to curses that spontaneously manifest within a person, due to the ambient magic in the Sea of Thieves.
The potency of curses also seems to be as varied in their effects as the curses themselves. According to Nine Cat Nura, curses get “meaner” with time. That is to say, if a person steals a cursed artefact, the longer ago the curse was placed upon the artefact, the worse and more “potent” the curse’s effects will be on the thief. It can be speculated that the artefacts which Rathbone stole from Tribute Peak had their Shores of Gold curse placed upon them by the Ancients long before other treasures in the Sea of Thieves had their Shores of Gold curses placed upon them by the Ancients. This could be because Tribute Peak, and its vaulted chamber, seemed to possess treasures that were far more personal and significant to the Ancients than their other treasure. The Shores of Gold curse of the artefacts from Tribute Peak was therefore meaner and more potent than the Shores of Gold curse of treasures from elsewhere in the Sea of Thieves.
However, Rathbone’s Skeleton curse lends weight to a perspective on how the “potency” of a curse operates differently in curses that spontaneously manifest. The potency of a spontaneously-manifesting Skeleton curse, for instance, seems to be dependent not on how old the curse is, but rather on the strength of the feelings that cause it to manifest. Given that Rathbone was willing to die to satiate his feeling of greed, this suggests that his feeling of greed was so strong that it caused him to become spontaneously cursed by an extraordinarily potent Skeleton curse, turning him into a Skeleton Lord. It can be speculated that the spontaneously-manifesting Skeleton curse was able to replace the Shores of Gold curse which Rathbone had possessed previously, because the Shores of Gold curse, whilst extraordinarily potent, was not as potent as Rathbone’s spontaneously-manifesting Skeleton curse. This would suggest that, when a person becomes afflicted with two curses, the more potent curse is the one which the person becomes afflicted with. Curses do not seem able to combine, nor coexist alongside each other, within a person.
It must be acknowledged that the Skeleton curse is the only curse confirmed to be able to afflict a person either by spontaneously manifesting within them, or by them stealing cursed treasure. However, whether this is only true in cases where a spontaneously-manifesting Skeleton curse specifically transforms a person into a Skeleton Lord, is unknown. Whether other curses besides the Skeleton curse can also occur in this two-fold rather than one-fold way, is likewise unknown. Nine Cat Nura also says that curses can change the people they touch in their very soul. This was confirmed in an interview with Mike Chapman, the design director for Sea of Thieves: Skeleton Lords tend to be an exaggeration of everything they were, at their core, when they were human. This would explain why, when Rathbone became the Gold Hoarder, his obsession with gold over everything else became exaggerated.
This theory would be consistent with other sources of canonical lore. In the journal of Flameheart Jr., it says that the Gold Hoarder was “driven by greed to the extent that he patches up his immortal frame with jewels and gold”. This proposition is supported by the Sea of Thieves Origins: The Price Of Gold comic. In the comic, the Gold Hoarder’s right hand is clearly made of bone instead of solid gold, unlike how it is consistently shown in all of his in-game, in-concept art, and in-model models. This indicates that the reason why the Gold Hoarder has golden body parts is not because he is simultaneously afflicted with the Shores of Gold and Skeleton curse, but because he repairs parts of his body over time, which would include his right hand, using gold.
In light of these assumptions, there could be three possible explanations, as far as can be speculated, about why there are Gold Skeletons in the game. The first is because the curse that turns a person into a Gold Skeleton is a unique variant of the Skeleton curse; both the Shores of Gold curse and Skeleton curse, combined into one curse by the Ancients. The second is because a Skeleton became afflicted with the Shores of Gold curse, or a person with the Shores of Gold curse became afflicted with the Skeleton curse, but the potencies of both curses happened to be equal; since no one curse was potent enough to replace the other, both curses coexisted alongside each other within the person. The third is what @Lleorb said: the Skeletons melt down gold and adorn their bodies with it, like armor. However, all these theories are unsubstantiated by evidence from canon, and so it therefore remains unclear whether curses are able to combine, or coexist alongside each other, within a person, or not.
