no. any existing observation fields are tied to the observation at that point. your project setup only defines the fields that must be present at the time an observation is added to the project.
if in doubt, you can always create a dummy project and some dummy observations and do an experiement.
another option you have is to set up a unique project for each season + year that you do monitoring, and then group those projects under an umbrella project. this way, each project can have unique fields. the complication with this is that if you have members or journals, etc., that may make it cumbersome for folks to join each individual project, plus an umbrella project.