Post: #2

And which is that 'given list'? I can't find any packet guide telling that packet 0x91 request anything or expect anything to be sent back... to be 'changed'.

Packet 0x91 only means 'Login': http://ruosi.org/packetguide/index.xml#client91
It checks that the 'connection' is made by a uo client (it could be telnet, or http, but reached this point only clients can pass).
It checks for a login success (ie, wrong password, banned account, already using...) returning 0x82 (PacketLoginError) if something is wrong.
Packet 0xb9 is sent with the allowed flags for this client: http://ruosi.org/packetguide/index.xml#serverB9 (Maybe this is what 'changes' ?)
Then packet 0xA9 is composed, sending the characters for this account: http://ruosi.org/packetguide/index.xml#serverA9 (Maybe this one is changing too):
-This send the characters you already have, but it already send the [STARTS ], the starting points you can choose when creating a new character, it also send some more account related flags.
And, after all of this, if everything was OK packet 0x82 is sent with BYTE 0xFF, which means success.