To make sure I'm reading it correctly, you want to insert rows from UserRank into UserRankHistory that don't already appear in UserRankHistory. If I'm interpreting it correctly, something along these lines should give you what you're after:

WITH cteNewRows AS (

SELECT UserID, RankName

FROM dbo.UserRanks

EXCEPT

SELECT UserID, RankName

FROM dbo.UserRankHistory

)

INSERT INTO dbo.UserRankHistory(UserID, RankName, CreateDate)

SELECT UserID, RankName, GETDATE()

FROM cteNewRows;

This is pretty simple, but because I don't have table DDL and sample data, I have to call it untested. Hope this helps.