You can use it at any point
I didn't know that, I just copy-paste them from a post by M@T but they worked for me. I'll try them with the change you pointed.
FFFB0000 = Select and FFF30000 = Select+Start, right?
Code by elixirdream (GBATemp.net)
Changes PKMN Data While Marking Them
Go to PC-Deposit Box-Select PKMN-Marking-Confirm to apply the following codes
Max Happiness
1206F04A 000020FF
1206F04E 00007338