If a ANSI 600 gate valve was initailly manufactured say for example in 1990, the maximum design pressure at 100 Deg F would have been 1440 psig.
However, now, API 6D reconizes that a ANSI 600 valve is rated for 1480 psig at 100 Deg F.
If I bought a used valve that was originally a 1440 psig valve and recondition it, can the valve be re-rated to the new 1480 psig?
By the time you recertify the valve with an auditable and approved ISO9001 QA program, and satisfy an API Inspector, chances are that it just won't be worth the time and expense.  Especially not for just a 40 psi increase. 
Part of the process is that you'll have to convince the inspector that the USED valve hasn't lost any strength through corrosion or erosion.
I have known guys who just barely avoided going to prision for "recertifying" used valves. 

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