Position changes in the timeline are likely due to an editing decision that affects them. For instance, if only an audio segment of a linked A/V file is deleted, the video will move. If clips are grouped or multi-selected, changing one may change the others, etc.
That's why it's important to Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, the AS choice is available by clicking the caret to the right of the Save Project button on the toolbar. That way you're protected, and shouldn't lose anything.
If you spot an unwanted change, immediately press <ctrl-Z> to backtrack, step-by-step. Or use Menu EDIT | UNDO.
Timeline clips are never replaced spontaneously here. Editing decisions may override them, and the Replace Clip feature can only utilized intentionally.