I was thinking about this earlier. Being a 'newcomer' here the following thought came to mind:
Themes: Existing XF2 themes will likely work but will need minor tweaks and changes to ensure they are 100% correct
Addons: The same as themes, they may need a bit of an update to be compatible, but not by any means a full rewrite
I am basing this on the theory that similar would of probably happened between XF1.0 vs 1.1 vs 1.2 , although I was not really here then (take no notice of my join date) so I cannot be sure, however logic dictates to me as this is a "mid point" (2 X something) version not a "major full" point version (1 something vs 2 something) this tells me that the updates for third party things would not create too much of a headache for both authors of said resources and for those using them.
Well that is my 2¢ on it!