The date has been moved back. The Form 1040X Amended Return is now supposed to become available March 4. But since the update usually occurs overnight Wednesday into the wee hours of Thursday AM, it may actually be March 5. Rarely, a form can be released earlier than shown in the table. No one can know for sure--only an expected date. It's been moved back a couple of times already.
TurboTax Form Availability Tool
In the meantime, do not make any changes in your original return. Also, if you are due a Federal refund, it's recommended to wait on the refund to arrive before amending. Or if you owed taxes due, to wait until the return processes.