@mz2 Thanks, that works, don't know how I missed that! Might be helpful to have a shortcut to change the citation style in the context menu that appears upon right-clicking a citation though, just for ease of discoverability.
Also, the autocompletion/autofiltering of the citation style selector seems a little buggy, or at least a little different from the way it works in other programs (including Papers). For example, say I'm looking for "Nature Nanotech" - normally, I would expect that as soon as I start typing, the list should be immediately filtered based on the initial string - so that first I see all those cite styles that start with "N...", "Na...", "Nat..." etc.
The way it seems to work in Manuscripts right now is that it only filters the list of citation styles if a substring of what I have typed matches a complete name of a citation style - in this example, it only filters the list of journals once I have typed "Nature", which makes the process a little slower.