Your query is not pertaining to bonus under the POB Act. It is partaining to bonus linked to your target, I suppose.
Bonus linked to target / production is depending upon the agreement you had with your employer. Abiding the terms of agreement, both the parties to the agreement are duty bound and a party who is aggrieved upon failure of another party to perform his duty, is entitled to legal remedy.
You need to take legal advise form legal practitioner upon giving him all the facts and figures, your agreement and other papers etc.
If your query is regarding bonus associated with targets, then the question of being legal is not appropriate. Performance Bonus and Incentives are completely decided by each company on their own terms. You can ask a question that is it fair, but again the answer to this will be given by individuals own perception.
Sorry to disappoint you but this is the way it is. Every company will state at the end of the letter (may it be bonus or incentive or whatever) that any changes in the policy will be informed to you, so you cannot win over this.