Article 4 of the 25th amendment of the US Constitution:
Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President
provides for the replacement of a President even if the President is unable or unwilling to acknowledge his inability.
This section is usually discussed in terms of sudden presidential illness or injury.
However, the section as written does not use language such as "has become unable", or "is now unable". Rather, it simply says "is unable"
Could the VP and Cabinet say, in effect, "We've just come to the realization that the President is not able, and never has been able, to discharge the powers and duties of his office."?