Reorganisations of share capital: alteration of rights
TCGA92/S126(2)(b) covers the alteration of the rights attaching to different classes of share. For example, a company may have issued both ordinary shares and convertible preference shares which can be converted to ordinary shares. A conversion of the convertible preference shares to ordinary shares would be a share reorganisation.
In strictness TCGA92/S126(2)(b) can only apply if there is more than one class of share in issue before the reorganisation. You can accept it will also apply if there is only one class of share before the reorganisation.