Update query on millions of rows fills the transaction log

Question

I need to update millions of rows as part of my next release, but doing so fills the transaction log and fails. I have a few ideas but I'm not a SQL expert so I'm sure there will be gotchas that I'm not aware of.

Pertinent points:

1. I need to hand a script over to the operations team so need a T-SQL method with no manual intervention.
2. Apparently the transaction log gets recycled every 15 minutes. (I've thought about writing a loop with a try-catch with WAITFOR DELAY '00:15:00' in the catch block like below)
3. (EDIT) I can't modify anything except the data.
4. (EDIT) It's a simple update changing a foreign key column to a different existing key.

Thanks,

Phil

DECLARE
    @AffectedRows int

SET @AffectedRows = 0

WHILE @AffectedRows < @RowsToUpdate
BEGIN
    BEGIN TRY
        BEGIN TRAN
        -- Do some updates  
        SET @AffectedRows = @AffectedRows + @@RowCount
        COMMIT TRAN
    END TRY
    BEGIN CATCH
        PRINT ERROR_MESSAGE()
        WAITFOR DELAY '00:15:00'
    END CATCH
END

PRINT @AffectedRows

Answer 1

In the end the example I had already written worked best; a transaction log full error gets caught in the catch and 15 minutes is long enough for the log to be recycled.

DECLARE 
    @AffectedRows int 

SET @AffectedRows = 0 

WHILE @AffectedRows < @RowsToUpdate 
BEGIN 
    BEGIN TRY 
        BEGIN TRAN 
        -- Do some updates   
        SET @AffectedRows = @AffectedRows + @@RowCount 
        COMMIT TRAN 
    END TRY 
    BEGIN CATCH 
        PRINT ERROR_MESSAGE() 
        WAITFOR DELAY '00:15:00' 
    END CATCH 
END 

PRINT @AffectedRows