Hi, the same thing happened to me, joined company, got taken over and forced to sign new contract.
I asked HR and other people.
Some points to help you out:
1. Once you hand your notice in, there is little a company can do to punish you (their ultimate sanction is job termination but you have nullified that by telling them you no longer want to work for them. Few companies will litigate, it is difficult and expensive and what would they gain or win?)
2. It is better to leave on good terms.
3. Even though your notice period is in your contract, often you can negociate a shorter period.
4. The HR department actually told me that they would want to amicably resolve the notice period issue rather than try to force it.
5. A colleague left at the same time. He was already on 3 month notice. He got the notice period he wanted - 6 weeks.
6. A freind pointed out that it actually good for you, the company must give you 3 months notice and MUST obide by it. In other words, if they make you redundany, you get 3 months to look for a job while they still pay you.
In other words, do not worry about it too much. You boss has actually indicated that the company would not force you to serve 3 months notice. In reality, there is nothing they can do. You could hand in your notice and never go back into work - what are they going to do? sue you? Worst they can do is not pay you. If they do force someone to go to work when they do not want to.....they will probably not do much work, be disruptive, may steal IP, may sabotage stuff, etc In others, it is not a good idea to force someone to work if they do not want to.
You can hand in your notice and put in writing the date you are going to leave. If the period is reasonable
eg 1 month or more they will probably let you have it.