I wonder (since the Tom/Mary Big Finish episodes are set between
The Armageddon Factor and
Destiny of the Daleks) if, taking Mary's failing health into account, they wrote/recorded a proper final scene/regeneration for Mary (in the last one she did before dying, yet to be released) to become Lalla. It would be a fitting - and poignant - thing to include in the 50th Anniversary festivities.