We have a special crossgrade deal for owners of the UAD versions. You can get a native version of any UAD plugin you own for $49, or you can crossgrade two or more for $99.
For now, you can't do it the other way around (Native to UAD). UA is the exclusive retailer of all UAD plugins, so it's up to them to create their own crossgrade policy, and so far they haven't.
If you'd like to proceed with a UAD to Native crossgrade, please send a message to our Sales Support team with a screenshot of your UAD license manager and let us know which license you need added to your account. We will then add the licenses to your account and you will be able to pay for them by selecting them on your Manage Registered Plugins page.