Orbit change
What happens when you change one part of an orbit?
The shape changes, not just the speed. Slow a planet at one point and it drops onto a longer ellipse that swings closer in and climbs back out. Speed it up and the far side lifts away. It does not spiral into the Sun unless almost all of the sideways speed is gone.
A slider is a thought experiment
Nothing in space grabs a planet and changes its speed by hand. What space does is push very gently for a very long time: tides, another planet’s tug, a rare impact. Every one of those shows up the same way — as a different-shaped orbit, not as a faster or slower planet going the same path.
To actually hit the Sun from Earth’s distance you would have to shed almost all of the sideways speed. Anything left is enough to miss.
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What can really change an orbit
Tides towing the Moon outward a few centimetres a year. Drag on anything low enough to touch an atmosphere. Repeated tugs at a simple period ratio — the gaps in the asteroid belt. An impact, which changes an orbit all at once but only by the momentum it delivers. On the scale of a planet, that last one is disappointing: mass for mass it is much better at changing spin than orbit.
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On the hub: Big questions · Orbital velocity simulator