Can a corporation grant security over inventory that changes constantly, like stock in a warehouse?

Yes, and this is one of the situations Ontario's Personal Property Security Act is specifically built to handle well. A security agreement covering inventory as collateral generally extends automatically to new inventory as the corporation acquires it, and releases items as they're sold in the ordinary course of business, without the parties needing to sign a new agreement or file a new registration every time the stock in the warehouse changes.

This works because of the PPSA's after-acquired property feature, combined with the practical reality that lenders financing against inventory expect the specific items on the shelf to turn over constantly — what the lender's security interest is really attached to is the category of collateral described in the agreement (inventory of a certain type), not any one specific item that has to remain physically present. Buyers who purchase goods from the corporation in the ordinary course of its business are generally able to take that inventory free of the lender's security interest, which is exactly what allows the corporation to keep selling its stock normally while the security interest remains in place over what's left and what comes in.