This is one of those weird old holdovers from the beginning of Premiere that doesn't make a lot of sense anymore.
I remeber a long time ago where a lot of the sequence settings couldn't be changed after the fact (such as resolution!) This seems to be the only option we are still locked out of which slows down some workflows.
The workaround has always been that to change from a stereo sequence to a multi-channel you have to use a multi-channel seqeunce preset or create one, and then paste your sequence from an old one into the new multi-channel seqeunce. It's a strange and infelxible part of the interface that gets in the way.
Request: Allow audio channel format to be changed after seqeunce created. Getting tired ot jumping through all the hoops when I just want to change to a multi-channel seqeunce from a stereo one.