It appears they can continue to rely on the S-3 filing until they have to re-assess the float when the 10K is due. If the float falls below the $75 million at the time the 10K is due, they cannot use the S-3 registration to sell any more shares until such time as the float goes back above $75 million.
"Of course, pursuant to Rule 401 under the Securities Act, registrants are also required to recompute their public float each time an amendment to the Form S-3 is filed for the purpose of updating the registration statement in accordance with Section 10(a)(3) of the Securities Act — typically when an annual report on Form 10-K is filed."