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According to the feature documentation for Record Templates, there is a default behavior by field type when applying a template to an existing record, as follows:
I suggest allowing these behaviors to be modified per template, through a similar screen as relative date settings can be. As an example, Single Select fields fall in the 'non-empty, non-array' category, but I often need to change one of these fields (e.g. Status) along with other changes to the target record. Assuming the categories must stay the same, the following options would be very helpful: