IDNA2008 should be backwards compatible. I can try to explain it in a practical example:

DENIC was the first registry that actually used IDNA2008 - at a time, where not even libidn2 officially included the changes required for it. This was mainly due to the point, that the German Latin Small Letter Sharp S ('ß') was treated differently to other German Umlauts ('ä', 'ö', 'ü') in the original IDNA spec: It was not punycoded, because the nameprep already replaced it by 'ss'. Replacing 'ß' with 'ss' is in general correct in German (e.g. if your keyboard doesn't allow to enter 'ß'), but then 'ä' would have to be replaced by 'ae', 'ö' by 'oe' and 'ü' by 'ue' as well. 

Punycoding 'ä', 'ö', 'ü', but not 'ß' was inconsistent and it wouldn't allow to register a domain name like straße.de, because it was translated to strasse.de. Therefor DENIC supported IDNA2008 very early to allow the registration of domain names containing 'ß'.

The only thing I'm aware of in this situation is, that previously straße.de was translated to strasse.de, while with IDNA2008 it's being translated to xn--strae-oqa.de. So people that have hardcoded a URL containing 'ß' and who are expecting it to be translated to 'ss' would fail, because with IDNA2008 it would be translated to a different ASCII-hostname. But those people could just change 'ß' to 'ss' in their code and everything would work again.

On the contrary, people that have registered a domain name containing 'ß' in the meantime couldn't access it right now by specifying the IDN version, because it would be translated to the wrong domain name with the current Python IDNA encoding. So the current IDNA-Encoding should be upgraded to IDNA2008.