Question

Assalaam alaikum. Could you please tell me if reading a certain way during a passage of the Majestic Qur'an which refers to Paradise and Allah's Most High's reward and so on (and may we be of it's recipients!) and then reading a different way during a passage referring to Allah Most High's punishment (and may we be preserved from it!) is part of Tajweed? May Allah reward you for your help.

Answer

Wa alaikum assalaam wa rahmatu Allahi wa barakatuh. 

There is nothing required in tajweed as to changing ones reading when reading an aayah referring to Allah's punishment or an aayah of Paradise or mercy, but there is the sunnah of our beloved Messenger, , that he would stop at an aayah of warning and seek protection with Allah from the punishment, and stop at an aayah of mercy and ask Allah for His Mercy.

Reciters with the ability to make us feel the meaning of Allah's words tend to change their voices at the aayaat of punishment to convey the warning, and at the aayaat of mercy to make us feel the desire for Alllah's mercy. 
The Prophet,  was asked: which people  had the best voice in the Qur'an and the best recitation.  He said, "He, who if you heard him recite, you see that he has fear of Allah."  Related by Ad-Daarimi, and graded as saheeh to others by Albani.  
This is not a requirement in tajweed, but is a very good quality to have, may Allah grant us this ability.