The new tonemapper shouldn’t have that large of an effect on your project(at least in this example). It encodes the scene referred linear color in a wider gamut and gives more room for varying display types when it converts back to the user’s display in sRGB. If anything, your colors should be a *tiny *bit different in saturation(with the exception of the blue primary issue in current ACES), whites brought down a bit so you get more room, and blacks brought up a little and potentially clamping if you don’t follow standard Albedo color ranges.
4.15 did bring a lot of post-process changes though, most notably the bloom threshold being more physically accurate. It’s possible the earlier version of your map was relying on a warm bloom to fill the scene, and with the new threshold the brightness values don’t contribute to the bloom so you’re left with the cool temperature scene. I’d look at and compare the post-process settings to make sure the color grading is identical(including the look-up table if there was one before). For a quick fix, you could look at the Temperature control in the post-process to bring back some of the warmth.
Before you modify anything though, I’d also make sure that your viewport is set to default settings(in viewport, Show > Use Defaults), your exposure is using the provided values in the post-process(in viewport, Lit > Exposure > Automatic(Default in-game), and your editor scalability settings is at Epic(above viewport, Settings > Engine Scalability Settings > all to Epic). I run into a lot of issues at work where people accidentally or for some unknown reason have certain things disabled in the viewport, so the first thing I always do is reset their viewport and make sure no low-quality settings are what’s breaking anything.
Edit: Also make sure that all textures carried over. See if the Skylight was using a texture or if it was capturing the scene. Same for the LUT, ambient cubemap in post-process(shouldn’t be used anyway), etc. Another test you could do is while in the old version, start disabling lights, color grading, etc. and see if you can replicate the look in the new version. That might help you track down where something changed in the conversion if something just broke.