To find the probability that a surveyed voter plans to vote to reelect the mayor, we can use the formula for probability:
\[ P(\text{reelect}) = \frac{\text{number of voters who plan to reelect}}{\text{total number of voters surveyed}} \]
In this case, the number of voters who plan to reelect the incumbent mayor is 255, and the total number of voters surveyed is 500.
Substituting the numbers into the formula gives us:
\[ P(\text{reelect}) = \frac{255}{500} \]
Now, we can calculate this:
\[ P(\text{reelect}) = 0.51 \]
Thus, the probability that a surveyed voter plans to vote to reelect the mayor is:
\[ \boxed{0.51} \]