Theme Upgrade from 2.9.1

Hi there,

What should I expect upgrading WordPress from version 2.9.1 to the current (3.5.1), if the theme is old and haven’t bee updated – my client is using that one – http://ww.wp.xz.cn/extend/themes/unspeakabledogness.

What should I expect to encounter ? How can I check the level of compatibility before updating ?

Thanks,
Amit