The annoying thing about shapechanging in 5e is that if you don't have a special ability, you need a 9th-level spell just to take on a humanoid form. 4th-level: polymorph into a beast. 9th-level: polymorph into pretty much anything. Nothing in between. It's something that really irritates me. There needs to be some spell in the middle that opens up the options a bit, for dragons and everyone else.
Well, perhaps, but remember, spells are designed with player characters in mind (and rightly so*).
First off: thank you for reminding me Polymorph isn't good enough to turn into a humanoid in this edition.
Then: there are bigger beefs to burn (does anyone say that?) than providing you with "story capabilities" like the one we're discussing here.
*) NPCs can always gain special abilities. Not requiring NPCs to play by the PC's rules is perhaps the most fundamental improvement of 5E compared to (mainly) 3E, in my opinion.
The Polymorph/Shapechange spells must first and foremost fix all the playability and balances issues of old editions. We all know what this entails, with the various fixes and limitations that got quite out of hand.
Restricting Polymorph to beasts beautifully fixes most of the issues, since it stops you from overshadowing the druid, stops turning the Monsters Manual into the Sears Catalog Of Overpowered Forms, and still allows "baleful" polymorph attacks.
Restricting Shapechange... is simply not needed when it's a level 9 spell. We all know all rules go out the window once you reach level 17!
What you ask for is simply something I fully understand and accept is not part of the "basic" rules. Any "in between" spell at perhaps spell level 6 or 7 will simply have to be so complex, for relatively little gain, that not having it in the PHB is justified in my opinion...
Zapp
PS. Besides, anything requiring Concentration is a no-good solution for my purposes (if a shapechanger couldn't blend in and join the allies in combat without running a significant risk of losing its assumed form each time it takes even a single point of damage, it simply cannot do so)
The one hour duration is a less critical limitation, assuming the NPC gets unlimited castings of the effect. Obviously getting a single level nine slot for Shapechange is not nearly enough for this purpose; what I need is (if we limit ourselves to 5E mechanics currently existing today) nothing short of the Change Shape ability currently given to only a small selection of very do-goody (and therefore uninteresting) critters.