The DM is free to change their mind regarding villain and season at any time.
Reading the introduction to Waterdeep: Dragon Heist, it is clear that the intent is for the DM to choose a season at the outset, so that the DM can immediately start describing the weather and citizen activity in Waterdeep that is appropriate to the season, in order to make the game more immersive for the players.
The villain of the adventure is nominally tied to the season the DM chooses, but:
You can swap out one villain for another at any time. For example, if
you decide halfway through the adventure that because of how the story
has progressed Jarlaxle Baenre would make a better antagonist than
Xanathar, you can make that change on the fly and run subsequent
The adventure unfolds in a particular season depending on the villain
you choose at the outset. If you switch to a new villain midway
through the adventure, don’t change the season to match unless the
characters take enough downtime for seasons to change naturally.
But what if I don't want to choose a season right at the outset?
This is totally within DM prerogative. It just means you won't be able to be as descriptive when describing the city to your players at the outset.