Thanks for putting this forward. You’re right that this can’t be done currently, but it would make a great addition, so I’m sure we can look at this.
I’m thinking from a stock item, you can simply pick another stock item and enter a “quantity multiplier” to link the two together and this would result in being able to be swapped “both ways”. The quantity multiplier would be useful if, for example, you need to use more hops when they’re whole-leaf versus when in pellet form.
I’m also linking this to Improvement suggestion to pre-brew stock check - on the fly changes to batches as the two could work well together and feed from the same “substitute” information.
If anyone else would find this use, please give this a vote using the button in the top-left.