Each release we will try to update what theme and template tag changes have occurred.  We will strive very hard to make sure that any changes we make, will not break in your own custom themes…   However, new features may require you to make a similar change to your custom theme in order to get that functionality…

So for 5.1.0, please refer to the following codex article on the theme and template tag changes:

http://codex.simple-press.com/codex/installation/changes/theme-and-templates-5-1-0/