A new property is needed since the old identifier P906 will still be useful as it is used by many third parties (and will be for some time). It could however get an constraint saying that items with that property should always also have this new property.
Sadly there does not seem to be an easy way of looking up the new identifier based on the old one that does not involve downloading large datasets.
The formatter URI for RDF resource (P1921) should probably be https://libris.kb.se/$1 (the canonical URI), I haven't checked if it supports content negotiation but e.g. .ttl is reachable under https://libris.kb.se/tr574vdc33gk2cc/data.ttl. The domain in the proposal is likely too narrow as the library will use this identifier for DVDs, editions of works and possibly other concepts such as events
|Open||None||T199977 Test Federated search Wikidata SPARQL with TORA sparql (LIBRIS)|
|Stalled||None||T200065 Define Libris and Tora as endpoints in Wikidata|
|Stalled||None||T200067 Define Libris XL as an endpoint in Wikidata|
|Invalid||None||T201268 Get Libris <-> Wikidata in LOD - Linked Open Data Cloud|
|Resolved||Salgo60||T200708 Property proposal/Libris-URI|
|Stalled||None||T200710 Libris XL: is there an easy way of looking up the new identifier based on the old one|