When you edit an existing message, it will not resend to the orders you already sent the message to, even if you adjust and edit the When parameters and an order qualifies for the message again. However, if you create a new message, it can send to orders you may have already targeted with any previously created messages.
If you decide to create a new message, and want to avoid resending it to previous customers, you could use the parameter "For Orders Between the Dates" found in the Advanced When filters of your messages. These parameters are an optional configuration that can be used to specify only sending to orders between specific dates. 

The date to the right is the start date, and the date to the left is the end date. You can read the dates from right to left as orders placed on or after DATE 1 and orders placed before DATE 2. Specifying DATE 1 ("Orders Placed on After" DATE 1) will effectively only target only new orders coming through after today's date. The second date, or "For Orders Placed Before," would most likely need to be set to a date in the future (ex. Dec. 31st 2099), to effectively target orders placed "on or after" X and onwards to the future (2099).

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