Well that all really depends on how close the lettuce will be to you HID system and how high the fixture is hanging above your "flowering plants...". Even with these things unknown i still do not expect you to have a problem switching to the "warmer" spectrum of light. Generally lettuce bolts due to temperature and to a lesser extent hours of light. this in mind i would be more worried about all of that excess heat your HID system is putting out. i also want to give you a heads up on a new indoor growing system that i had the pleasure of conducting the horticultre trials for. unfortunately due to a contract i had to sign i must be vague. but i can say that with this new system it will cut your operating cost way, way down . it is also very very easy to optimise for specific plants. i do not work for them just ran the trials and reported my findings. i am simply someone who loves all things plants and likes to share info that might be helpful to others. as so as their website is up (should be soon) i can share more. send me a message if this at all interests you.
david