Fixed selling plan prevents items being added to the order

[MirraMichael]

When a selling plan with a fixed pricing plan exists on an order, no products are allowed to be added to it. 

If you try to add a product (not a custom product) to an order containing any item with a fixed selling plan you get the error: "You cannot add items to this order because it contains an item with a fixed selling plan."

This limitation has existed for as long as selling plans have been available but it's a huge restriction that limits the flexibility of try before you buy orders (and any other order flow that uses fixed selling plans). 

• Admins can't edit orders when a customer requests changes or makes a mistake
• If an item is out of stock, it can't be swapped for a different size or style
• The new exchanges features that Shopify has rolled out can't be utilised for the same reason, despite try before you buy orders being one of the most obvious use cases for this functionality

Are there any plans to support this for orders with fixed selling plans?

[trinifirestorm]

Having this same issue, customer pre-order items , she wanted a bigger size in one of the items and now it's not editing at all. Please advise this fix.