In login dialogs show if current account is work or personal and add a checkbox to choose
My work and personal accounts have the same email address and since you "simplified" the login UI it is cumbersome to switch between systems, e.g. Office365 (work account) and Windows Dev Center (personal account). If one does not remember which account type was used on the last login (current account) there is only trial and error to find out.
Tedmond Lasseter commented
I second this. Right now I can't just log into my old one drive with my live account. Nice going on the confusing change and hats off.