Feature pack upgrade?
Voted best answer
You can refer the 800xA product lifecycle document. It shows clearly upgrade path SV3.1 onwards.
Here is the link.
Your question is a bit confusing.
First of all you can not UPGRADE directly to any feature pack from any revision.
-->If you are in 5.1 RevX you can only UPDATE to feature pack (using FUT) as below..
Eg: If you are on RevB, you can update to FP3 or FP4 but not FP1 or FP2.
--> If you are on 5.0SP2 RevX, you need to UPGRADE to 5.1 RevX (Using System Installer) and then UPDATE to a feature pack as mentioned above