There is a difference between upgrading or crossgrading another Steinberg product and a competitive crossgrade.
As Starsprinkler says, nothing happens to your qualifying product when you buy a competitive crossgrade to a Steinberg product. Steinberg give you a discount because you already have a similar product that means they’re less likely to sell you the Steinberg product at full price. You are free to keep using the qualifying product after crossgrading and it will be fully functional in any Steinberg software.
When you upgrade or crossgrade a Steinberg product, the old licence is replaced by the new one on your eLicenser. Normally, this doesn’t matter as with one or two exceptions you can run an old version of a Steinberg product with a licence for a newer version. HALion 6 is one of those exceptions - you need to contact Steinberg support to get a free HALion 5 add-on licence based on your HALion 6 licence if you want to run HALion 5 or earlier). Licences for a bigger product normally allow you to run the smaller product - so you can run HALion Sonic 3 with a HALion 6 licence, for example.
The one situation I can think of where a Steinberg crossgrade does restrict your options is crossgrading from Cubase Pro to Nuendo. In this crossgrade you give up the qualifying Cubase licence and you cannot run Cubase with that Nuendo licence. These days Nuendo is a superset of the same version of Cubase Pro, but the Nuendo product cycle lags the Cubase one, so whilst Cubase 10.5 is out, Nuendo is still on 10.