Set Form to Pristine without clearing data

What you're looking for is myForm.markAsPristine().

At the moment I can suggest two possible solutions. First one is really close to what you've suggested, as form's reset method has following signature and accepts form value as a first argument:

//@angular/forms/src/model.d.ts:
reset(value?: any, {onlySelf}?: { onlySelf?: boolean; }): void;

In the submit handler, we will capture a copy of the last state:

const { myForm: { value: formValueSnap } } = this;

And do the reset itself:

this.myForm.reset(formValueSnap, false);

Another option from the times, when there was no possibility to reset form, is to create a helper method, which will mark each control as pristine and will keep the data. It can be called in the same submit helper in place of the resetting.

private _markFormPristine(form: FormGroup | NgForm): void {
    Object.keys(form.controls).forEach(control => {
        form.controls[control].markAsPristine();
    });
}

Link to the updated plunkr.

If you happen to be using Template-Driven forms and you have something like this in your component: @ViewChild('myForm') myform: NgForm;

I've found that the markAsPristine() is a function on the form property of your form. So it would be this.myform.form.markAsPristine().

Just thought I'd add this in case others come across markAsPristine() as not being defined.