Sidegrade in videogames means something that gives a different playstyle but is not worse nor better than the weapon it originated from
For example: High damage but low fire rate weapon and low damage but high fire rate weapon are both sidegrades to each other
"I can't aim properly with this weapon!"
"Then get its high fire rate Sidegrade."
"I can't kill them in time!"
"Then get its high damage Sidegrade"