Once you have the switch set up, there is a smartapp that lets you have flipping a switch (virtual or physical) change the mode or run a Hello
Home Action (which can change the mode and do other things at the same time). See #31 and 32 under Convenience:
Then once the app is set up so flipping that virtual switch changes the mode (you’ll need on and off, off to switch it back to your regular night time mode), add that switch to your Watch Movie activity.
Finally, change whatever you have set up to have your sensor turn on your basement lights NOT run in the new movie mode. (If it’s already set up to only run in Night mode, you won’t need to change it. But if it’s set up to run at a specific time of day, or without restriction of any kind, you’ll need to add a mode restriction.)