I think this affects artists the most. They'll be wondering how they should draw Tenshi from now on.
Not really. Zent's reply summarizes it enough. New official art, whether it's new artists, outfits, hair styles, body figure, etc; everything strictly increases the pool of design material people pull from to create derivative works. People concerned about using any specific designs will pull from that material, but generally people draw from their understanding and envisioning of a character based on the combined material of that character (even including other derivatives). Adding new artistic detail, even when some aspects are changed to be different than previous material, doesn't erase the previous stuff. This is particularly important for Touhou; malleability of character design even in official works is part of why the designs are so appealing.
Additionally, people already portray Tenshi as young. No new artistic details are really even added here to pull from. The slightly derpy face she's making is legitimately the most detail that's being gleaned.
Besides, this does paint events of SWR in a new light - Tenko acted like she did because she literally IS a spoiled child
This was explained in her profile though? And the fact remains that she's arbitrarily old anyways; her behaviour is all you have to talk about apparent age. Whether her body might have stopped aging or not is neither here nor there, especially since anything that's not a normal human can be designed to look however old.
Anyways, the main reason I say this doesn't matter (not that people "should" stop caring about it) is because artistic detail in Touhou generally does not infer setting detail. How an artist chooses to portray a character does not "make" them something different. If e.g. Tenshi is supposed to be young, she is young because the writing and author intent shows as such, and the art is informed from that depiction or direction, not the other way around. This art by itself
doesn't actually inform people that Tenshi is meant to be young. Without the other existing cues already suggesting that, there's no reason to say it's anything but artist choice. She could be given massive tits just because the artist wants to; it does not matter. At least stuff like random massive breasts gets to bring up the question of how it got past ZUN.