There is no clear way to distinguish that art is related to PriConne but not PriConne Re:Dive, or vice versa, unless it includes a character new to Re:Dive or is in-game official art that was not reused (quite a bit of the art was reused.) In that sense it's a somewhat similar situation to sekaiju no meikyuu and its immediate sequel, which has no tag.
Since the original PriConne wasn't very popular and has been shut down for years, an argument could even be made for an alias if anyone thinks that's more appropriate.
The original Princess Connect takes place in our world in 2033 with your character starting a VR game and meeting many girls both inside and outside the game. The main heroines are Yui, Hiyori and Rei.
Princess Connect! Re:Dive takes place after a "bad end" of the original storyline where the villain succeeds in making the VR game reality and erasing everyone's memories, so they don't remember they once lived in our world (but the helper goddess occasionally manufactures dreams of you being lovey-dovey with the characters' real world selves as a reward for bonding with them.)
The main heroines in Re:Dive are Pecorine, Coccoro and Kyaru, none of who existed in the original game or its story. Most of the other characters that were in PriConne, including Yui/Hiyori/Rei, were brought back for Re:Dive (or are gradually being added in over time) using the same sprites and portrait art.
Thus, it essentially is a sequel, not just a remake. The problem is that due to all the shared characters and reused art, there's very few ways to distinguish art of the two games besides applying logic like "was this art created before Re:Dive launched in February 2018" and "did these characters exist in the original game," and it raises the question of whether there's much of a point in having separate tags.
Edit: It looks like there's a grand total of 9 images on Danbooru that predate Re:Dive on the final page of the princess connect! tag.
