The work is yours, and you can make it available in any way you see
fit, possibly with different licences. So yo can perfectly well change
your mind, and change the conditions of the licence under which you
make your work available. You can even have different licences
available at the same time, giving different rights to the user,
possibly for different prices.
Of course, if you decide to use a stricter licence than the one you
used so far, people who acquired your work under the more permissive
licence will keep benefitting from it, whatever it permitted.
If you decide on a more permissive licence, people who got your work
under the former stricter licence do not benefit from the new
permissivity, unless the get a copy of the work under the new licence.
But if the new licence is CC-by-sa, it is probably easy to acquire
that new copy (usually ... I know of counter-examples).
So the short answer is: you are doing fine, and no problem is to be
expected, other than readers not knowing there is a more permissive
However you have to make sure that you did not sign any contractual
document that forbids you to distribute under another licence ... for
example with whoever printed your thesis.
Typically, when you work with a publisher, you will transfer to him
(part of) your copyright, and you can no longer distribute the work
under a new licence, without his permission.
Note: Of course, your very wide freedom regarding the way your
work is made available does have some legal limitation, not
necessarily expressed by copyright law. For example, a discriminatory
licence that prohibits sales to a specific class of people might get
you in trouble.