Victo,
I think looking for a problem where none exists!
Most of the talk here has been a discussion about whether your FAA Airmans certificate is restricted by just the restrictions on the face of your JAA licence, or also by the restrictions in your home country's licensing regulations.
But if I understand your position correctly, you have no restriction on night VFR flight in your home country! There is no day only restriction on your licence and no day only restriction in your leglisation.
As I understand your position, you got these restrictions removed when you got an IR (as well as getting IFR privlidges). If for example you let your IR lapse, you lose your IFR privlidges, but the day only restriction shouldn't reappear!
Check the wording on your licence. Bose X gave it to your earlier. It will most probably say
"the privileges of the this licence may be excercised at night"
Therefore you have no restriction on your licence, and no restriction in your leglisation. So all the talk of restrictions on your licence vs restriction on your licence + restrictions in your licensing regulations isn't relevant, as you have no restriction in either!
So where would the restriction to day VFR only on your FAA Airmans cert come from?
dp