I suppose you have some auto-generator for unique 16-char codes.
Let's say it's encapsulated in function dbo.GenerateUnique16key.
Then you should have something like this:
Create TABLE dbo.AccountIdentifiers (
TwentyTwo char(22) NOT NULL PRIMARY KEY,
Sixteen char(16) NOT NULL DEFAULT (dbo.GenerateUnique16key)
When new records arrive insert 22-char codes into this table.
Table will generate 16-chars unique codes and assign them to imported 22 char codes.
Then mapping (converting) will be easy once and forever.