Back when Guild Wars 2's second season of post-launch content was coming out in 2014, they introduced a new map. And for the first few weeks people were debating if the massive vines on it were actually growing or not, or if it was just that their animation gave the illusion of it. Some suspected they may simply be expanding with each patch, but there was no noticeable change on patch day immediately after the server updated either.
Within the month though, people had actual time lapses taken on the map, showing a barely perceptible pixel-by-pixel growth of them, at regular intervals, which led to them reaching all the way across the entire map by the time the story reached a climax.
If the Thargoid sites were changing in a similar manner, we'd have that same kind of footage by now. The ED community do LOVE their time lapses. And there are plenty screenshots available of the same Thargoid sites over extended periods of time.
Sure, they may be growing into some kind of Thargoid ship, but we've seen the kind of timescales the Thargoids operate on. It's likely that we'd see it take years for any perceptible growth to take place, without some kind of major update by Frontier to accelerate that. As it is, they're static.
Any changes we have seen to known sites have been as the actual planet generation model changed over the period we've been observing them.