what are the rules for changing the affirmative sentences to negative and vice versa.

would the meaning remain same while changing the form from affirmative to negative and vice versa.

There are no specific rules to change an affirmative sentence into negative and vice versa. However, there are certain rules to attempt  the grammar topic "Transformation of sentences" as it mandates that you perform such conversion without changing the meaning .

For example: 

Brutus loved Caesar. (affirmative)

Brutus was not without love for Caesar. (negative)

Some rules to covert an affirmative sentence to negative are given below for your reference:

1.  Sometimes → Replaced by → Not + always.

 Affirmative: Radha sometimes visits me.

 Negative: Radha doesn’t always visit me.

2. As soon as → Replaced by → No sooner had ----- Than

Affirmative: As soon as the child saw the police, he ran away.

 Negative: No sooner had the child saw the police than he ran away.

3. Only→ Replaced by → None but

 Affirmative: Only God can help us.

 Negative: None but God can help us.

4.  Both----and → Replaced by → not only ---- but also.

Affirmative: Both Divya and Dipika were excited.

Negative: Not only Divya but also Dipika was excited.