Can I upgrade my KeePass 1 (KDB) database to KeePass 2 (KDBX)?

Yes! It's possible to upgrade from KeePass 1 (KDB) to KeePass 2 (KDBX), however, you cannot do this in the Strongbox iOS ~~~~app at the moment.

You should be able to do this in KeePassXC software which is free to download for Mac/Linux/Windows. Please see their docs here:

Which password database formats are compatible with KeePassXC?

KeePassXC currently uses the KeePass 2.x (.kdbx) password database format as its native file format in versions 3.1 and 4.x. Database files in version 2 can be opened, but will be upgraded to a newer format. KeePass 1.x (.kdb) databases can be imported into a .kdbx file, but this process is one-way.