One of the motivations for development of TDT and UDT is that you don't necessarily get an opportunity to do such self-modification beforehand,

Basically this, except there's no need to actually do it beforehand.

Consider the set of all possible source codes an agent might have. This set is partitioned in two: those on which Omega rewards you (where you both one box, and your source code is such that Omega can tell you will), and those on which Omega punishes you (all others). Call the former set A, and the latter set B.

Agents are not guaranteed to start with a source code in set A, some start with source code in B. (Maybe they are classically trained causal decision theorists? Maybe they are skeptical about UDT? Maybe their programmers were careless? Who knows!) The point is, there comes a time in an agent's life when it needs to grow up and move its source code to set A. Maybe it does not immediately self-modify to directly do UDTish things on Newcomb-like causal graphs, maybe it self-modifies to self-modify before being asked to one box.

But it is crucial for the agent to move itself from set B to set A at some point before Omega shows up. This is what I mean by step 1.