Takhalluṣ is a smooth movement from one subject to another through a link the audience can follow. Iqtiḍāb describes a shift not supplied with the same kind of evident semantic bridge.
أَمَّا بَعْدُ
Now, to the main matter.
This is a familiar sermon transition, but its mere appearance does not automatically make every subsequent subject logically integrated. Likewise, when “this” introduces a new theme, one must examine what it refers to and how the sections relate.
The advanced question is therefore not merely “did a transition occur?” but “by what link did it occur, how clear is that link, and is it adequate to the speaker's purpose?”