Marvelous Mario’s Inc. et al. v. St. Paul Fire And Marine Insurance Co. provides authority for the principle that an amendment to plead discoverability is available at any time:
 The plaintiffs’ pleading is silent as to discoverability. Recognizing the gap in their pleading, the plaintiffs have moved for an order allowing them to amend their pleading to plead discoverability. St. Paul takes no position on the motion. Rule 26.01 of the Rules of Civil Procedure, R.R.O. 1990, Reg. 194allows for an amendment to be made at any time, even after the conclusion of trial: Hardy v. Herr, 1965 CanLII 225 (ON CA),  2 O.R. 801 (Ont. C.A.), at para. 2. Discoverability was an issue thoroughly canvassed at trial. I see no prejudice to St. Paul in granting the amendment. Accordingly, the plaintiffs’ motion to amend their statement of claim to plead discoverability is granted.
All of that said, it’s worth noting that the facts setting up a discovery argument are properly pleaded in reply.