If you bought Pro from the official site Lime Wire » Official Website & Free Download, BitTorrent « (payment to LimeWire LLC shows on your CC statement) & your Pro benefits are still current, then you can update to LW 5 Pro without paying. If your Pro benefits have expired then you won't be able to upgrade to LW 5 Pro for free.

So if you bought 6 months worth of Pro, you download free installers for 6 months after purchase. If you bought Extended Pro then you can download free installers for 12 months after purchase. If you bought the 'one-off' Pro, which includes no updates or support then you won't be able to download an installer for free.

If you bought Pro from a scam site then you won't be able to download an installer for free.

Make sure that you save your Pro installer! If your Pro benefits aren't current any more & you decided to download the free LW 5 - that would uninstall your current version. If you didn't have the installer saved then you'd lost that version

There's no way of getting rid of the update nag - it's just a PITA that's here to stay if you choose to use an older version