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.