User Name Capitalized

[mark350]

Unknown to me, my programmers had my forum set up where people were not able to choose a user name with capital letters. I have two questions:

1. Is there a setting in vbulletin admin to define user name parameters in regards to capitalization?

2. Is there any way I can take a user name such as "mark350" (lowercase m) and change it to "Mark350" (uppercase M) without causing any problems to any of the previous posts made by "mark350"? ... and can I do this in such a way that it would automatically change the user name "mark350" to read "Mark350" where it appears on various posts?

Thanks in advance for any help.