Is there an idiom for "Your decision can change at the last moment"

Solution 1:

Just to cite a saying with another lady, you may say:

It's not over until the fat lady sings

• used for saying that it is still possible for a situation to change, (also at the very last moment.)

(Cambridge Dictionary)

Solution 2:

Here are some common idioms that imply that people's choices can change at the very last moment:

Play it by ear

This implies that a decision will be made when it needs to be made, not before. It is often used to imply that the decision will be based on information that is not currently available, but may be available when the decision needs to be made. As in: “I understand,” said Mr. Hardy. “How much time do we have?” The sergeant looked doubtful. “Don't know. We'll play it by ear.” (from the Hardy Boys 27: The Secret of Skull Mountain By Franklin W. Dixon).

Not carved in stone

-or-

Not set in stone

This implies that a decision has been made, but may be changed. As in: Mind you, my decision is not set in stone. This idiom is used in other situations (such as rules, relationships customs, memories, schedules, etc.) The first use I could find where this phrase was used metaphorically is by Charles Dickens in Oliver Twist: But they had recognised him, and he them; and their look was as firmly impressed upon his memory as if it had been deeply carved in stone, and set before him from his birth.

Up in the air

This implies that the decision has not be made, even though the information that needs to be considered is available. This idiom can be used for decisions that may be difficult to make, or the person making the decision needs more time to think. As in: Do we vacation at the beach or the mountains this year? I am up in the air about this.. I can also mean that one is feeling, well, high, as this excerpt from the autobiography of Nat Love: ...I was feeling up in the air caused no doubt partly from the effects of love... right before he decides to try to lasso a steam locomotive.

Solution 3:

This tautology is also common:

it ain't over till it's over (TFD)

The final outcome cannot be assumed or determined until a given situation, event, etc., is completely finished. Usually used in reference to organized competitions, such as sporting events, political elections, and the like.

Edit:

As noted in the comments, it's worth pointing out that this expression was coined by Yogi Berra, an American baseball player who was famous for his pearls of wisdom, a.k.a. "yogi-isms" (wiki).

Solution 4:

Don't count your chickens before they're hatched

meaning:

Don't make plans based on future events that might not happen

Solution 5:

English has several idioms about the possibility that the situation might change unexpectedly, but not many that focus specifically on people changing their already-made decisions. I think the closest analogue to your idiom may be

It's (a woman's) prerogative to change (her) mind.

Like the Turkish phrase, this likely originally referred specifically to decisions about marriage, since in common law an affianced man could be sued for breach of promise for backing out of the engagement, but betrothed women generally weren't similarly legally constrained. See Wikipedia for a discussion.

Nowadays, it's just used to mean that people can change their minds, and the phrase can change to suit the situation: "it's my prerogative to change my mind" or "it's a person's prerogative to change his/her/their/one's mind" and so forth. Some examples of the extended usage of the maxim:

To be sure, many of the government's more publicly successful ventures, free trade being a notable example, were surprising (and somewhat hypocritical) carry-overs . . . . Governments, after all, retain the prerogative to change their mind.
—Dean Oliver, "Foreign affairs and national defence", Canadian Annual Review of politics and public affairs, 1994, ed. David Leyton-Brown

It is the referee's prerogative to change his mind provided the next shot hasn't been played, and the video playback seems to vindicate the referee's decision to review the shot.
—Anonymous comment on "Williams Blasts Williams", Snooker Scene Blog, 11.9.11

If Google has taught us anything over the years, it's that it has the prerogative to change its mind about what ranks best in search results.
—Amanda Pressner Kreuser, "This New Study Cracks Google's Algorithm for 2018 (Here's What's At the Top of the List)", Inc.com, Jan 30, 2018