Plans Change

Plans will change as implementation progresses.  This should be minimized
but is practically unavoidable.  Rather than a single transition from planning to
execution, it will probably bounce back and forth, with each planning and execution
phase getting smaller as the implementation zeroes in on the final form.

You can’t know the best approach for anything in infinite detail when you start
for any practical problem (this is engineering)

Planning is something that will need to be done periodically as information
becomes more clear. Also, the project will never feel “fully done” -- there will
always be additional layers of polish that could be added.  Polishing is also a
good analogy -- rough and then gradually finer.

The goal of a good plan is not to become set in stone, but to “see around the corner
 and anticipate as many of these issues as possible.  And also to serve as a living
document and record to determine how well the initial plan matched the final product.

The project never really leaves the planning phase until implementation is complete.