Since you already own HLC, you would start here -
https://www.lonewolfdevel.com/Classic-Licenses/Manage - and link your HLC licenses to an HLO account (you can create a free account to start). The Lone Wolf store says to do this before making any purchases.
If you want to use HLO, you'll need a standard level subscription to HLO. To share your content with your players, you'd need a patron level subscription. And there's a nominal fee, I believe it's still $10, to sync all your existing licenses with HLO. After that, anything you own and any future purchases will be available in both HLC and HLO.