upgrade from 11.3 HF7 to 11.5.1 HF7

I'd refer you to the release notes, especially the new features and new behaviours section - this'll help to remove any unexpected changes.

Main difference between 11.3. and 11.5. will be no more Http Class and Local Traffic Policy in its place. The upgrade can/should upgrade these for you but if there are complexities e.g. certain regex strings, then it may not do so. Also, any http class commands in irules are no deprecated and will need the irules to be rewritten.

Yes, license activation would be good practice before any upgrade.

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