Anything you have obtained through the in-app store would be stored in your account, and can be accessed again by logging into that account throgh the in-app store system.

It's not completely clear what update you're referring to, but I assume this is about the major program update from generation 2018 (25.x) to generation 2019 (27.x) of Music Maker. This would have resulted in you being logged out from the in-app store for technical reasons. (This isn't something that would usually happen with minor program updates.) So, manually logging back into the store under the correct account is a must, I'm afraid. After that, you should be good to go.