If the old username is JohnSmith and you upgrade that to a Microsoft id during the WX or 8.1 free upgrade then all settings except the password fields will be auto-transfered into the new username. Email is an issue if its not able to sync with the current rules... basicly any Microsoft email will do but not linux selfhosting or some other back yard job.
The new password needs to meet the min requirements so: 8 characters; at least 1 cap letter, a number and standard US keycodes (i.e. not $, * or @)... yes if you have a us keyboard then you can have $ but if you ever get a Australian/ UK english system or most Windows phones then you will not be able to log into your account unless you also install/ run the US keyboard layout as well as the one for your country because these keyboards don't have the $ key.