Now a days people are moving from manual testing to automation testing and if reverse case then.
What to do?
It is honestly a decision that you alone have to make. It all depends on what you choose your path to be for the future.
Judging by the tone of your post you really are disappointed on the change, thus I would recommend talking to your manager and expressing that and maybe you can come to a better fitting resolution.
If that does not pan out, even though you are also thinking about it, it might be time to move on to a more suitable position of your choice - automation.
People tend to classify "Automation" == Good, "Manual" == Bad. Putting aside your personal preferences, life is not always so black and white. What is this manual project about? the latest software controlling the spacecraft to mars and you are responsible for initial exploratory testing? A disposable handheld device? Some projects may still be interesting and valuable career-wise even when tested manually.