Once you register, you can't change stuff (I'm 99% sure).
What you do is simply re-register under the pseudonym (or with any changes you wish to make)....and it doesn't matter that there are two registrations, since it's you in both cases...but even if say, 30 other people registered your songs, it still doesn't matter, until there is *a question of ownership*.
It kinda' sucks, since you have to pay again....but that's how it is.
You may be able to make changes BEFORE a registration is processed, but you have be quick about it and get in touch with the L of C.
I recently registered two songs electronically, and accidentally "Closed" the upload option before uploading my lyric sheets (l had already uploaded the audio files). I called them the next day and the guy opened the upload for me and I was able to add more stuff....but of course, my registration had not yet been processed.