Can I update a sourced field via csv import? If not if I remove sourcing will other records lose their values?

If its sourced, then you can't update it unless you're updating what its sourcing from. If you change the type from sourced to stored, then the records will lose their values as they aren't sourcing anymore. You can change it to stored and then do your csv import and then you should be OK but it won't source anymore and you'd have to fill out the field another way for future records.