What does it mean that feelings will change in autumn?

It means that people's feelings will experience some changes with the passage of time and the change of seasons.

Feelings will change in autumn, which means that with the passage of time and the change of seasons, people's feelings will experience some changes. Autumn refers to the process from summer to autumn, and autumn is usually considered as the season of change and departure. This sentence means to remind people that feelings can change, just like seasons. As time goes by, people and their partners will face various challenges and changes, and their emotions will fluctuate accordingly. This means that feelings may experience some difficulties, changes or separation, but it may also mean that feelings will deepen and stabilize to some extent.