Username with changed...

[Callid]

Cyllyd.

Sounds Welsh

Replace any e or i with an y, and any o or u with an w. Furthermore, replace any k, t and p with g, d and b (respectively).

[miakakiri]

myagagyry

Woah, unpronounceable mess.

Idem Callid-san.

[Aokochan]

awgwchan

Write each syllable twice.

[Callid]

Calcallidlid.

@ Aoko:
It should have been "Awgwchan", not "-chwn"

@ miaka:
Why is that unpronounceable? My-a-ga-gy-ry is perfectly possible.

Replace any character with the character (left) above it on the keyboard, and the first few in the first row (q-u) with the ones in the bottom row, and i with y, u with w and p with " " (space).

[Aokochan]

Qoiodqh (like this?)
@Callid: Oops you're right! *edits*

Replace all the consonants with the letter "d".

[breva]

ddeda

Change a for @ , i for 1 , e for 3 ,and o for 0

[miakakiri]

m1@k@k1r1 (modified 1337-sp34k?)

@Callid-san: I think you have an odd keyboard... space is on a level below the bottom row of my keyboard, and the top row reads 'qwertyuiop'.

Write your username in 1337-sp34k.

[Aokochan]

40|<0(|-|4|\|
idem

[breva]

8|23\/4

Write your username in katakana

[miakakiri]

katakana: ミアカキリ

Since my name is made up of actual Japanese names/words, I could do it easily enough in hiragana as well.
hiragana: ã

[Callid]

Dodrika. Or "dorrika"? How shall I treat the small tsu?

@ miaka:
First, I made a typo, it should be "o with w", not "u with w". And yes, that's the top row I assumed, and yes, space is below the bottom line for me as well. That's why I, O and P have special rules; they are not the ones above them. So, basically, the rules are:
1. Every letter is replaced only once.
2. Replace with letter above (to the left).
3. If not possible (i.e. character is in top row), and character is between Q and U (American keyboard), replace with respective character in the bottom row (American: Z to M).
4. Replace I with Y.
5. Replace O with W.
6. Replace P with " "(space).
And yes, the last three don't care for the location of I, O, P, Y, W and space.
So, on an American keyboard, "Callid" becomes "Dqooye", while "Tropic" would become "Bvw yd".

Use the rules above!

[miakakiri]

jyqiqiyvy

It's not an 'American keyboard', the layout is called 'qwerty' for how it starts. And 'above to the left, but replace top row with bottom row' still works. You'd replace 'i' with ','; 'o' with '.' ; and 'p' with '/'. Using 'y' as a replacement for 'i' gives 'y' two uses--it's the letter directly up-left of 'h' and would therefore be the proper replacement for 'h'. Similarly, replacing 'o' with 'w' means that 'w' is either 'o' or 's'. This is confusing.

Reverse the capitalization of your name.

[Callid]

cALLID.

First off, calling it "American keyboard" is useful for distinguishing it from other QWERTY variants. Also, it makes it clear what I'm talking about even to people who do not know the American layout.
I didn't want to include non-letter characters, which is why I didn't use "," etc.
And yes, Y, W having to uses was intended (also, the way I wrote it makes some letters not be used at all).

Use a different colour for each letter!

[breva]

breva

Add "san" after each vowel

[Aokochan]

asanosankosanchansan :p

Insert the name of your favourite character in the middle of your username.