Hi,
the points raised by you & answers
1: Rules for notice period- there is no defined rule or law which governs this. Companies have norms as per the criticality of role and position. Even within the same company, the provision may be different for different levels & departments. However, no company can hold an employee for an unreasonable period (which has to be defined in context of the role) of time after he/she has expressed desire to leave ( unless bound by a bond or such clause in which case there might be financial implications or non-compete restrictions)
2. waiting period/getting a good job: again depends on the role & the urgency for filling the position. There are companies who respect the notice period requirements as they understand that an employee who can shortchange previous employer can do the same to them. At the same time, if they are in a hurry, they give a very lucrative offer which can offset the notice pay loss or even agree to take over notice pay. from the current employer also, al lot depends on your relationship with your superiors and with HR. If you have been a great employee, the organisation would also want to keep good relationship with you and therefore either waive off the notice period and charge you in lieu or even waive off the notice pay.
3. Company's right to change: the offer/appointment letter given by any company contains suitable disclaimers & caveats which allow the company to change the rules of employment as long as they are not specific to any employee and are not biased ( which can be termed as discrimination) . All such changes if approved by the senior management & informed to all employees with suitable notice are valid. It is theoretically possible to challenge such changes if a large majority of employees oppose it. However, in practice it rarely happens in private organisations and employees typically fall in line.