Well, I took the plunge based on the operating instructions posted on
help.boox.com, which indicated that the frontlight control still worked the way I like it. However, as it turns out, this is no longer the case since 3.5. The frontlight controls have been removed from "settings" and are now available only in the control panel. Instead of the previously separate controls for the "warm" and "cold" LEDs, the sliders now control intensity (brightness) and color (with cold at the "-" end and warm at the "+" end). By default these are automatically reset to "-" when the Boox goes into standby. The right edge slider, which previously controlled warm LED intensity, can now control various parameters. The default and the closest to the previous functionality is for it to control frontlight intensity. However, since the color temperature resets to cold every time the device is turned off, the right edge slide effectively controls cold light intensity now. The previous ease of use for me is broken, since I have to set the color temperature in the control panel every time I turn on the device instead of just sliding up the right-hand edge.
P.S. In the Boox Feedback section I asked about installing 3.3.x instead.