You can:
Change one letter - e.g. Start to Stare
Add one - Start to Starts
Remove one - Start to Star
Move one - Start to Tarts
You cannot move more than one letter, or combine any of the moves (e.g. you cannot change the S in Start to a Y and then move it to make Tarty).
The last word on the old thread was Mile, so I’ll go with:
Mill