Impossible in alcatel systems, the TRX band gives the list of frequencies you can choose, during frequency allocation.
there will be radiations, but weaker. The pattern might eventually be modified as well.
The dipoles of your antenna are the size of the wavelength / 2. So putting 900 into 1800 dipoles is possible : the dipoles are wavelength / 4 -> still proportional to the frequency.
But the pattern will be modified accordingly, and I suspect the gain will be lessen (probably divided by 2, as far as i recall).
And it is not dangerous. It is never dangerous 🙂 electomagnetic waves will always find a way to the air (“freedom !” i heard them say, sometimes), and they’re never going to blow up anything.