uncheck "Generate automatic URL alias" for existing nodes of specific content type

how can I uncheck "Generate automatic URL alias" for all existing nodes of specific content type?
I would like to keep all existing aliases created with a pattern, and so I need to uncheck "Generate automatic URL alias" for all nodes of that content type.
In the future, I want to set a new pattern for new created nodes and have an new pattern set for these.
Any tips?
Tanks in advance