I don't know how a future loss is treated from a tax perspective (does the lost investment generate a capital loss?) but overall this seems to be similar to standard tax accounting.

Assuming an arms-length transaction: this would be like taking shares you own in one company and exchanging them for shares in another company. Typically you would sell them for money in-between but even if they were traded directly you would need to recognize the capital gain at the price you traded them, based on the current value of what you got for them. This would be applicable to the current taxation period, and if the new shares tank it could generate a capital loss.