first of all you should check if this MM326 message does not come because of a program bug,
see OSS note 1709285 - Incorrect output of message MM 326 for profit center change
In that case you would not need to change the message customizing.
In OSS note 160970 - Change valuation class: production order/purchase order
it is described for what reason the message is issued. The risk is always up to you if you decide to change the attributes of a message. You have to evaluate the given info with the situation in your system/with your local design. If you can organizational wise secure that you do not create inconsistencies after making it warning only.