The taste of the milk may change due to the storing process. For example, there may be excess lipase in the milk due to the storing and causes the frozen milk to taste soapy after being thawed. The way to go about this is to scald the express milk first before freezing (you can refer to this blog post on how to scald the milk: http://simplyrebekah.com/2010/05/03/excess-lipase-scalding-breast-milk/).
As long as your baby accepts the taste, you need not think too much about it. Some babies would reject frozen breast milk because of the taste difference. So you can try feeding it to your baby to see if your baby has any reaction to it.