You could place the old girl under the new light at lights out with no issues, more light won't hurt it... only less. The HID takes some time to get up to full power (up to 20 mins) so it'll get a gradual introduction like a summer day.
Make sure you keep the new one further away than the old tho or you could get a surprise
The HID WILL make a difference in bud density and should help control stretch a little better even just from the heat the bulb puts out.
Congrats on the flowering with CFLs. I would love to try it but the room would be a problem.
And also on the new light, don't you love new toys... I mean tools?
