There is no discounted upgrade from the ‘SE’ versions of Steinberg plugins to the full version - you have to buy the full version at full price. (There was one historic exception to this rule, now consigned to history - Padshop Pro was available as a low cost Pro add-on licence to the version of Padshop 1 bundled with some Steinberg DAWs, as well as a full price product that did not require owning a Steinberg DAW that included Padshop).
If you are considering buying Groove Agent, consider whether the Absolute collection makes sense for you. Absolute 4 is Groove Agent 5, HALion 6 / HALion Sonic 3, The Grand 3, Padshop 2 and Retrologue 2, plus some expansions for Groove Agent, HALion / HALion Sonic and Padshop that would require additional chargeable licences if used by non-Absolute owners. You can buy Groove Agent and upgrade to Absolute later, but it is cheaper to buy Absolute as a single purchase. The full price of Absolute is less than the full price of Groove Agent plus HALion added together, so you still get a good deal even if you already have Padshop 2 and Retrologue 2 as part of the higher editions of Cubase 10.5 or Nuendo 10.3.