When interchanging sentences, you have to make sure that the meaning of the sentence remains unchanged. Positive, comparative and superlative sentences can be interchanged in the following way:
Positive: No other boy in the class is as smart as Kevin.
Comparative: Kevin is smarter than the other boys in the class.
Superlative: Kevin in the smartest boy in the class.
The first sentence compares Kevin to the other boys in the class using the positive adjective “smart.” The same meaning is conveyed in the second sentence by using the comparative adjective “smarter.” Finally, the third sentence uses the superlative adjective “smartest” to convey the same meaning as the first two sentences.