How to preserve manually defined folds when switching to expr within a session?

I use a plugin that uses expr foldmethod. From time to time I want the flexibility to manually define my folds. I just switch to manual by:
setlocal foldmethod=manual
and all those foldlevels calculated by the expr is still there, which is perfect!

However, when later on I want to use that expr mode to fold some newly added content up again, there seems no easy way. If just switch back by:
setlocal foldmethod=expr
I'd lose all my manually defined foldlevels! Is there a way to preserve my manually defined folds when switching back to expr?

Thanks.

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