CRA indicates that a gain from a trust disposition of QSBC shares can be allocated to a subsequently-added beneficiary

Where a discretionary family trust realizes a capital gain from the disposition of qualified small business corporation shares, there is nothing to stop it from allocating that gain under ss. 104(21) and (21.1) to a beneficiary (e.g., a "new" spouse) who is not added as beneficiary until later in the year (or, it would seem, in a subsequent year if there is an earnout so that recognition of the gain is deferred).

Neal Armstrong. Summary of 23 June 2015 T.I. 2015-0571801E5 F under s. 104(21.2).