Yes you can make amendments to a previously approved application. 

There are two types of amendments, non-material and material. Non-material amendments are the least significant form of change that can be made and should have no material effect on the planning permission. Material amendments, on the other hand, will have a material effect on the planning permission. 

Please see the guidance notes for Approving Non-material Amendments to an Existing Planning Permission. https://gov.wales/planning-permission-small-amendments-existing-permission

If the amendments you wish to make are consider material then you can apply to vary the plans condition on a consent. https://gov.wales/apply-planning-permission

If the amendment you wish to make is considered to be Non material then you can apply to make a Non material minor amendment on the existing consent. https://gov.wales/apply-planning-permission

Alternatively, if the proposed change is materially different to the original application a new planning application would be required. https://gov.wales/apply-planning-permission