You can actually (in principle) do an experiment for one of those.
If you had a huge, dense, thin sheet of matter in empty space you could cut a person sized hole through it then attach a cylinder to the hole and put a flat bottom on the far away end.
So on one side it looks like a flat tower coming up from a big plane and on the other side it looks like an empty well in the middle of a big plane.
There is gravity pulling yourself to the sheet from each side. So when you start out a few km above the well you accelerate towards the bottom of the well until you get level with the thin sheet of matter at which point gravity accelerates you the opposite direction it used to be accelerating you. (There can be some discomfort when you feet are on one side and your head is on the other, but 1g on each end isn't necessarily the end of you.)
However the well is just a few 10 m deep compared to the few km you fell through so you are still going quite fast when you hit the bottom of the well. Splat.
So if you accelerate at 1g in one direction for a few km then accelerate at 1g in the opposite direction for a few meters then hit an object at rest to what you were before you started then ... splat.
So that won't save you. You could try to make a larger acceleration to save you in a shorter time. We can't actually make a sheet of matter like that out of ordinary matter, so there is no experimental version of that.
And if you just tried to accelerate at a huge acceleration by using a rocket or something (not gravity) that only pushes on part of the person then the person's own body has to pull the rest of itself along to stay together, that will hurt and rip you apart. So that won't save you. In fact that is what happens when you hit the ground, so you are just spreading it out over a bit more space it's like landing on something soft or an air bag (which isn't soft as it inflates but it does exert force over some more distance).
Now let's look at changing your mass. This could happen too for one direction. For instance you might make a craft that was full of matter and antimatter (which both have positive mass) and you decide to let it annihilate into light which you send outwards leaving a much less massive craft behind. But realistically your mass could only get to a minimum size like the mass of a single neutrino, we don't know anything smaller than that to be left over. So that doesn't get you to 0 or get you large again.
Changing your mass makes all forces affect you easier (because they need to supply less energy to increase your momentum). So the air around you will push you hard. An atmosphere of pressure isn't a big deal to your normal self and your body is used to pushing itself outwards to fight that pressure. But now each of those bits of air is pushing just as hard but you react more extremely to it.
And since little bits of air push you ever so slightly differently you'll be thrashed around here and there. Air bumps into itself and moves around randomly, you are less massive so when one air hits one part of your body that part would go flying of the air is moving fast. That might hurt to have different parts of your body poked in different directions so hard, like being pelted with hail from all sides.
You won't be pelted from all sides equally hard though. The air on the bottom pushes you up a little bit harder than the air on the bottom pushes you down so on average you will feel a strong force upwards. Strong enough to support an equal volume of air against gravity and that air is much more massive than you. So this means you feel a very strong force upwards. You also have some friction which is upwards as well and could be just as strong as gravity used to pull you down so now there are lots of forces pushing you upwards. The friction is like a force that would have accelerated your old self at 1g upwards (assuming you were at terminal velocity) but it accelerates your less massive self a huge amount. And the pressure is enough that it could accelerate a you sized blob of air and you are much less massive than that so you feel a force strong enough to push up with a large acceleration (though if you were at terminal velocity it was the sum of these forces that countered gravity so this last is a minimum force and it is at most enough to make your old self go 1g upwards with that maximum achieved depending on your surface area and how smooth and flexible you were and such and and those are all properties from back all before your mass changed).
So you have a force upwards but it could feel like you are being slowed down by a person below you shooting bullets at you. Or with the speed you see air rush out an open airlock on a deep space spaceship you might see the air push your lower self up to your top self as you become a pancake with you body effectively putting up negligible resistance. Or your body might make air molecules bounce off it elastically and create this giant vacuum behind it as it shoots up.
And there are combinations maybe some air molecules can shoot through like bullets and other drag you along them like a Mac truck and others send you flying like a tennis ball being hit by a basketball. All depending on the exact speed it hits you and what your new body is like. And the bullets could cone from all directions.
And as your mass increases the air might stop going through your body after it already started going through. So molecules of gas could end up inside you with air in your organs and tissues. And it's all chance what happens
For instance if a very high speed molecule hits you right when your mass is very very very very small and your low mass body is rigid with a very high speed of sound then it might pick up such a huge speed in such a short period of space before it hits the next molecule that by then the mass is higher and so nothing slows it down much at all from that really large speed it got.
So there is a chance that right as your mass gets close to zero you start shooting off at near the speed of light and vaporize everything on your one way trip out of the galaxy. And die.
So you could die either way and I'm not even sure how to change gravity to be strong enough if you change gravity.