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.
Master Muir provides a helpful summary of amendment principles in Concord Adex Inc. v. 20/20 Management Limited:
 The law in relation to motions seeking leave to amend a pleading is well settled and was not seriously in dispute on this motion. The applicable principles are summarized in my decision in Greenwald v. Ridgevale Inc., 2016 ONSC 3031 (CanLII), 2016 ONSC 3031 (Master). At paragraph 21 of Greenwald I set out those factors as follows:
• the amendments must not result in prejudice;
• the amendments must be legally tenable;
• the amendments must comply with the rules of pleading;
• a motion to add a party must meet all of the requirements of a motion under Rule 26.01;
• the addition of the party should relate to the same transaction or occurrence;
• the addition of the party should not unduly delay or complicate the hearing;
• the addition of a party will not be permitted if it is shown to be an abuse of process.
It is useful to keep these in mind when considering whether a motion to add a party of the presumptive expiry of the limitation period.
There is also a reminder that standard of discovery applicable when determining whether to add a proposed defendant after the presumptive expiry of the limitation period is reasonable discovery, not possible discovery:
 With respect to all of these arguments it is important to emphasize that it is reasonable discoverability and not the mere possibility of discovery that triggers a limitation period. See Crombie Property Holdings Ltd. v. McColl-Frontenac Inc., 2017 ONCA 16 (CanLII) at paragraph 35; leave to appeal refused, 2017 SCCA No. 85. The proposed defendants appear to be holding the plaintiffs to a standard of perfection. That is simply not the test.
A limitations practice tip—when amending a pleading to add a new claim, it’s the filing of the motion record that stops time running, not service of the motion record. Master Albert recently made this point in Becerra v. Ronchin:
41) The limitations clock does not stop running until a proceeding is launched. In the case of a motion to amend a pleading, the proceeding is launched either when the motion record is served (arguably) or more correctly when the motion record is filed with the court. This is akin to the issuance of an action or application stopping the limitations clock where a plaintiff is launching a fresh proceeding. Service of an intention to issue a proceeding prior to issuance does not initiate the proceeding. Similarly, service of a notice of motion together with a without prejudice letter and without a motion record does not constitute the launching of a proceeding to amend a pleading in an existing action.
You can’t amend a claim to assert a new cause of action if the cause of action is statute-barred. The question is, when’s an amendment a new cause of action?
In Beauchamp v. Gervais, Justice Dunphy sets out the following test:
 The preceding authorities establish that in order to qualify as something other than a new cause of action the proposed amendments must, in substance, be: (i) an alternative claim for relief, or a statement of different legal conclusions based on no new facts or not going beyond the factual matrix from which the original claim arose; (ii) better particulars of the claims already made; (iii) a correction of errors in the original pleading; or (iv) the assertion of a new head of damage arising from the same facts. If the amendments cannot be characterized in one of these ways, the amendments should not be permitted, in order to not deny a defendant the right to rely upon a limitations statute.
This paragraph follows a lengthy summary of the relevant jurisprudence that’s worth reading if you’re considering the issue.