Remember that to engage a mode you have to disengage the existing mode
I'm not surprised you don't understand, I did not explain at all well! I will try again...
To change a mode the conditions must be right
both to engage the new mode
and also disengage the present mode.
On take-off the a/c is in the "Common" mode TAKEOFF; this combines vertical and lateral modes which can be disengaged separately.
The SRS mode can be disengaged in three ways.. (FCOM 1.22.30 P.47)
1. Automatically at ACC ALT or if ALT* engages to capture the FCU ALT.
2. Crew selects another pitch mode (e.g pull ALT knob, push V/S to level off).
3. Crew pulls SPD knob for selected speed.
Notice that selecting TOGA is not one of the three, so it will not change the pitch mode. The lateral mode will be NAV, HDG, or RWY TRK by now and because SRS does not disengage the lateral mode will not change.
Now imagine you are established on the ILS, and have a look at FCOM 1.22.30 p.51 for the disengagement conditions for LOC and G/S modes and you will find
"- Engages the go-around mode" in the list, so selection of TOGA makes the mode change to the "Common" G/A mode, giving you SRS and G/A TRK.
Hope that is a better explanation!