There is provided a security door leaf assembly, comprising a leaf frame structure, having at least a top rail member, a bottom rail member, a hinge stile member and a lock stile member; at least one inner leaf panel, removably mountable to a first side of said leaf frame structure; at least one outer leaf panel, removably mountable to a second side of said leaf frame structure, that is opposite to said first side; at least one first security-upgrade liner, and at least one first upgrade cavity, formed by said leaf frame structure, said at least one inner leaf panel and said at least one outer leaf panel, accessible via said at least one inner and/or outer leaf panel and adapted to removably receive and operatively affix said at least one first security-upgrade liner; and at least one first liner mount adapted to mount said at least one first security-upgrade liner in a predetermined position within said at least one first upgrade cavity.