The pair have were strong performers in last week's SANFL loss to North Adelaide, with Stewart bouncing back from injury with a 33-disposal effort.
Newton has been the most consistent AFL-listed performer for the Magpies this season.
The trio of inclusions replace Tom Logan, Sam Gray and Kane Mitchell - all smalls - for an important clash with the Demons.
Cassisi's final game for Port Adelaide comes on the back of three consecutive losses for a Power side looking to press back into the top four.
Ken Hinkley says the game is important for many reasons outside of farewelling an important servant of the club, particularly getting back into winning form.
"Taking the emotion [of the day] out of it, getting back on the winners' list is the most important thing," Hinkley says.
"We need to play well to make sure we get our season back on track, it's certainly not a long way off track, but the last couple of weeks have been a bit of a stumble for us.
"We want to play well first and then, hopefully, we'll deliver a performance Dom will be proud and part of to make sure he goes out a winner."
Port Adelaide plays Melbourne in AFL Multicultural Round on Sunday 20 July at 12:40pm.
Port Adelaide Round 18 AFL team v Melbourne
Sunday 20 July, 12:40pm, Adelaide Oval
FB: Cam O’Shea, Jack Hombsch, Dom Cassisi
HB: Matthew Broadbent, Tom Jonas, Jasper Pittard
C: Jared Polec, Travis Boak, Kane Cornes
HF: Paul Stewart, Justin Westhoff, Matt White
FF: Robbie Gray, Jay Schulz, Chad Wingard
Foll: Matthew Lobbe, Ollie Wines, Brad Ebert
Interchange from: Hamish Hartlett, Jarman Impey, Ben Newton, Aaron Young
Emergencies: John Butcher, Sam Gray, Tom Logan
In: Cassisi, Newton, Stewart
Out: Mitchell, Logan, S. Gray (omitted)
Milestones: Cassisi, final AFL game (retiring)
