The answer is partially yes and partially no:
1. Yes, you can change the tabindex
2. The discussion around type="email" is a little bit longer
is a new HTML5
feature. Therefor, only the browsers which have implemented HTML5 will render it as such. The older browsers (like IE7 and IE8) will simply consider those inputs as type="text".
As a result: the type="email" is a nice and good feature, as I don't understand why do you need to change it. See also:
Moreover, the old browsers will automatically consider that type as text, so no additional changes are needed for them.