Is it possible to migrate an SD card formatted as adopted storage to a new phone without reformatting?
Though you did not (yet) confirm both devices are rooted, this solution might be of interest for those where they are:

Following up to our adoptable-storage tag-wiki, you will find a link to How to decrypt adopted storage? Based on that, the following approach seems feasible:


From the original device, extract the encryption key file located in /data/misc/vold
Copy this file to the new device


There's a missing step, though: As the file has no fixed name, I assume this must be stored in some settings database. So that database entry must be found and added to the new device. On the other hand, the name might as well be derived from the label of the SD card it belongs to.

So basically, this answer might be incomplete – but the approach worth a try. If someone can provide the missing step, please do so in the comments and I'll integrate it here (with credits given, of course).