Huh what? Other than a few small changes, the rules for Arcana in the RC (pp.133-136) are identical to the rules for Arcana in the PHB (pp.181-182).
- Arcana Knowledge: Identical.
- Monster Knowledge: Identical except for DCs.
- Identify Conjuration or Zone: Identical except for DCs.
- Identify Ritual / Identify Magical Effect: Combined into a single "Identify Magical Phenomenon" block in the RC. RC also says you understand the effect/ritual's basic purpose and effects, which the PHB doesn't, but this is arguably just a better phrasing of RAI. Otherwise, they're identical except for DCs.
- Sense the Presence of Magic: Identical except for DCs and duration (reduced from 1 minute to standard action).
If you're referring to the "improvising" block, those are
samples, not rules. Improv is up to your DM and always has been. Personally, I encourage my players to use their skills, including Arcana, for all kinds of cool stuff.
The only significant change is to the DCs, which I embrace wholeheartedly. The old DCs were too hard, then too easy. The new ones finally get it right, and about time, too.