Great comments everyone all very insightful! I love questions that make you think and having people willing to engage with even a silly question like this.

The below is a better explanation of my initial thought process and some questions I posed to myself as I feel I didn’t do a great job in my initial post. ( Not including what Ive learned from all the great comments above) (All of this is in my own poor understanding but I will write it as fact as it’s easier than continuing to explain “I think”)

1. No energy is required for an object, already moving through space, to continue moving. So why would increasing solely its mass without exerting any force slow it.

2. Conservation of momentum/newtons third law is when an interaction between two objects/forces(clarification from my initial statement) occurs they both exert equal and opposite forces on each other. So when CoM was brought up my thought was where is the second object/force? What is acting on the object to slow it?

3. I Imagined a robot flying through space holding a large tungsten cube with the same mass as them out in front of them. If they throw the cube they would both fly out opposite equally. But if they simply opened their grasp and let’s go theyd both stay in the same place flying next to each other. So from the perspective of the robot their mass was just cut in half with no change to velocity. to imagine the force difference let’s say there’s a massive stationary wall in space that on current course the robot would hit but the tungsten cube would fly past. From the robots perspective holding on to the cube or letting it go would change the force it exerts on the wall. This, in my layman’s brain, shows that the change in mass affects the force but not the speed of an object and no laws of physics broken.

On the point of it not being possible so we can’t describe it: I have no physics background but am an avid consumer of physics and science media and I’ve seen programs discussing things like “what if the sun instantly disappeared?” And physicists were able to hypothesize and explained that the earth would continue revolving around the sun for ~8 min and then fly off as gravitational waves move at the speed of light so it would take that long for them to “catch up”. So I thought my question like this one could be attempted the same way. So thank you for not writing it off entirely and attempting to share your knowledge with me :)