Not sure if that's some nice new functionality.. here's what I think I know..
You can change your mind about your instrument choices, for a 14 day window of time after purchase. (that's defo true)
It's normal for people to think they want one piano on the day they purchase, only to find within that 2 weeks that instead maybe they'd prefer a different piano. That freedom to decide in your first 2 weeks is baked in to the offer.
Otherwise, don't worry too much if you think you messed it up - around that 14 day window you can always contact Modartt support for help ("I think I may have changed my instrument choice by mistake") - they will reset your final choice of instruments for you. If you're a bit over your 14 day period to lock in your choices, within reason I'm sure they'll make exceptions for you if you definitely feel you made a regrettable choice (like maybe a new piano dropped right outside that 14 day zone - and "Wha but I just locked in this other one! Damnit Janet!". Pianoteq has you covered.
Also - it can be overlooked, but within Pianoteq, going to "Help" then "About" will give you a one-click link on the resulting panel to 'update your license'. This just means that the data in your user area (in the Modartt website) gets final say and will inform Pianoteq (in the app) - and within Pianoteq you will then see the instruments in sync with what's in the user area. (takes a few seconds usually - NOTE that, it has historically been possible that it may require support to fix something rare going on with that - rare though if you're one of the lucky ones.)
Sometimes, when people buy a new piano, that's the way to make it show up in Pianoteq. "I bought a new piano but it doesn't show up.. what do I do?". Do that
Anyway - hope that stuff leads to an idea of a clue to a possible avenue towards a likely way forward along the bright path to a clear and open sea of possibilities and.. that kind of thing.
(caveats - I bought before any feature to remove or re-instate instruments existed.. and even now, it's only hearsay for I've never encountered such a beast on the high seas of my user area.. so I urge you to not take my word for it (mwaahahaha)). That's just to say - could be wrong about something above - and always love to learn and welcome someone else coming forward with the correct info if anything about the above is wrong.
Happy Pianoteq-ing!
Pianoteq Studio Bundle (Pro plus all instruments) - Kawai MP11 digital piano - Yamaha HS8 monitors