If you're the executor of the owner's estate, you shouldn't have any trouble doing so unofficially but just to be safe, it's better to get the letters of authority from the probate court first. If it's a security issue, how about contacting the local police and asking them if it's OK?
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I agree with Mr Reynolds. It is probably better to get the letter of authority by opening the estate in probate court, but if you are the named executor in the will and it is a matter of securing estate property from those who currently have access you are probably ok to do it now.
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Get letters of office then a courtesy call to the police documented in writing is what I have always suggested.
Then go ahead and change.
Act fast if the premises is unsecured. Things go downhill fast.