How do I make the Doctor regenerate?

In Lego Dimensions, once you finish the Doctor Who DLC, a message flashes up saying "The Doctor can now regenerate!" ...And then I didn't manage to read the rest of the message before it disappeared.

How do I unlock and use this ability? When I walk into the TARDIS, the panel on the left-hand side has a menu of Doctor regenerations - but they are all locked when I access it.

Solution 1:

From my testing:

• The Doctor will switch generations when he dies, in order. So if dying while set to the First Doctor, he will come back as the Second Doctor. If he dies as the Twelfth, he restarts and comes back as First.
• For a regeneration to appear on the menu in the TARDIS, the characters need to enter the TARDIS while the Doctor is in that form. So if you walk in as the Fourth Doctor, Fourth gets added to the menu.

Having done the above, if you make the game save (e.g. "Save and Exit" a level), the Doctors should permanently appear on the TARDIS's menu: