K 1.5.13 is the last version for the K 1.5.x series. Now that K 1.6.1 has been released development team support for K 1.5.x is extremely limited.
Your question is basically, "Can I upgrade K 1.5.11 to K 1.5.13 and still keep my customised changes?" Basically, the answer is yes but it depends on where you stored your customised changes. If you changed the default_ex template and saved your changes into that template folder then you will lose those changes with the upgrade (but you can always replace those files you changed from a backup copy after the upgrade). But the big problem with K 1.5.x series is that the template contains a mixture of PHP and CSS and the maintenance releases for K 1.5.x often changed the PHP code. If you use a template designed for an older version of Kunena you may have functionality problems.
This last problem that I mentioned has mostly been overcome by the new approach used in Kunena 1.6. This is, perhaps, one of most significant benefits of using K 1.6: customised changes can be retained from one version to the next.
The answer to your question "Can I upgrade K 1.5.11 to K 1.5.13 and still keep my customised changes?" should be yes ... but it depends on the changes you made and how you went about making them.