We had this situation in a game of Small World tonight. Seafaring Tritons took over the lake (using two race tokens) and then removed one of them during re-deployment. There was an adjacent Sorcerer player and we wondered if he could use their special ability to take over the lake. We ended up not doing it, but it seems to me that the Sorcerer's special ability would allow them to replace the single Triton there and conquer the region. Would that work?
No, you need to be able to conquer the adjacent territory. Only races with Seafaring can conquer the sea or Lake Regions. The rules state on the Seafaring ability (page 11):
As long as your Seafaring race is active, you may conquer the Seas and the Lake, ... Only seafaring races may occupy the seas and the lake.
On the Conquering a Region section (page 4), regarding what conquering means.
To conquer a region, a player must have available to deploy: 2 Race tokens + 1 additional Race token for each (cardboard piece) ... Seas and Lakes cannot usually be conquered.
Upon conquering a Region, the player must deploy the Race tokens he used to conquer this Region inside the borders on the map.
Following Conquests, showing that you must have conquered a region to occupy it.
Each of the newly conquered Regions must be adjacent to (i.e. Sharing a border with) a Region already occupied by his active race tokens, unless permitted by his Race and Special Power combo.
And finally, the sorcerer's ability (page 9)
Once per turn per opponent, your Sorcerers can conquer a Region by substituting one of your opponent's Active tokens with one of your own taken from the storage tray.