You can modify the plan as you wish, as long as the changes are within reason.

within 是介词，意思是"在...之内"，表示范围或界限。 reason 在这里是名词，意思是"合理的范围、正当的理由"。 within reason 这个短语表示"在合理范围内、不过分"，强调修改计划时不能超出合理的限度。