Try switching to Play mode, then go Play > Playback Template. Select one of the factory templates (HSSE+HSO (Pro) if you’re using Dorico Pro), then Apply and Close. You should now have playback. If any of the instruments sound wrong, it’s because Dorico has misunderstood the instrument definition in the MusicXML file. Switch to Setup mode, click the disclosure arrow to the left of the relevant player in the left panel, then click the disclosure to the right of the instrument that appears immediately below. Then click Change Instrument, search for the correct instrument and hit OK.
As to the crossgrade discount, yes, you’re eligible for it as long as you can prove that you’ve been paying for Sibelius Ultimate for a minimum of 12 months. If I’ve understood the small print correctly, the subscription product now known as “Sibelius” is not eligible - it’s a cut-price flavour of Sibelius with a limited feature set (in the same way that Dorico Elements is a cut-price flavour of Dorico with a limited feature set). It’s difficult for Steinberg to provide clarity over this, as Avid have renamed what used to be called “Sibelius” as “Sibelius Ultimate”, and all the people that have e.g. Sibelius 6, 7, 7.5, 8 etc. are eligible for the crossgrade discount.