Jquery ui effects filename have changed. For example change

footer|combine_script require='jquery.effects.blind'

to

footer|combine_script require='jquery.ui.effect-blind'

We had backward naming compatibility for 2.5 but not for 2.6.

the file admin/themes/default/uplodify.jgrowl.css was moved to themes/default/js/plugins/jquery.jgrowl.css

use this line to include the file in your template :

{combine_css path="themes/default/js/plugins/jquery.jgrowl.css"}

no changes for the javascript file

Following http://piwigo.org/doc/doku.php?id=dev:changes_in_2.5#add_action_buttons
http://piwigo.org/dev/changeset/23263

Themes have to handle two new template vars : $PLUGIN_INDEX_BUTTONS and $PLUGIN_PICTURE_BUTTONS
Plugins creators SHOULD NOT add <li> or <div> or <span> around their buttons, it's now handled by the theme.

TokenInput CSS was moved from admin/themes/default/style.css to themes/default/js/plugins/jquery.tokeninput.css. Needs to be explicitly imported with

{combine_css path="themes/default/js/plugins/jquery.tokeninput.css"}

allowCreation parameter was renamed into allowFreeTagging

SwitchBox are used for derivatives choices, sort options, LanguageSwitch, ThemeSwitch, etc.
Now the JS code is factorized in a function available in themes/default/js/switchbox.js
Usage :

{combine_script id='core.switchbox' load='footer' require='jquery' path='themes/default/js/switchbox.js'}
{footer_script require='core.switchbox'}switchBox("#sortOrderLink", "#sortOrderBox");{/footer_script}

The CSS is still provided by themes.