I haven't gotten to the point in the game you're discussing yet, but I am enjoying the premise of the game so far. I thought of a potential solution to the problem you seem to be having that might not require as much retconning/reworking of previous chapters. It seems part of the issue is that Julia is a forced LI, which would take a lot of rewriting to fix. Imo, trying to "fix" what is essentially the setup for the game that's given in the description on the first page to appease people who aren't into that premise but played the game anyway is probably a losing battle, in many respects. A more pressing issue (again, this is just my perspective having not gotten to that point in the game yet, so feel free to ignore it if you think I'm way off base), is that a lot of people seem to resent that they feel like they're being forced to cheat on Julia in order to choose another LI. Just make it so you can choose another LI without having to cheat.
If you still want to rework/retcon previous older content (including backfilling the game with more Julia content to make her more compelling as a LI), you'd still have the option to do that at whatever pace you feel comfortable with if you're convinced the number of new fans those changes would bring would be worth the effort of changing that existing content. In my mind, it seems like it would be most important to fix the situation that is bothering the most fans that you already have.
I've thought of two potential fixes to the forced cheating situation, which aren't necessarily mutually exclusive. The simplest is to just give the option to break up with Julia without having to cheat on her. Another possibility would be to have the option to open up their relationship before getting to the point of choosing to be with another LI. I know you're not wanting to make a harem game, but if you want to implement the possibility of dating multiple LIs at all, I'm not sure there's a better place to do it than when deciding whether to leave or stay with the default LI. This would obviously depend a lot on how you would end up writing/implementing the scenario whenever it would come up, but it's difficult to imagine that it would feel natural for that option to be available later in the game if it wasn't a possibility at this point in the story. Verisimilitude can be a tricky thing to achieve, but from a MC perspective, it tends to kind of break down in a narrative when a protagonist is required to make a sacrifice in a certain situation but isn't when that situation comes up again.
Great job and good luck
