Paladins need more magic.. and then they are more different from Warrior and fix this issue.
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WHM has 6 offensive abilities. ACN has more offense than a pure healing design, but it's not actually that obtuse. ACN also has two healing abilities (Physick and Resurrection). The main thing is that ACN has, though, is a mechanic to hijack for making it into a healer. You tick all the necessary boxes as soon as you hit 30: normal heal, burst heal, baseline heal, AoE heal, enmity reduction (innate), combat raise. The weaknesses are addressed at 35 and 40 with AoE boost and cleanse. At 45 and 50, you hijack another mechanic (Aetherflow) to carve out a niche.
To me, a smooth role change requires either innately-appropriate abilities or readily-repurposed mechanics. CNJ is as vanilla a caster as it gets. There are no special mechanics to hijack, so you either deal with the current ability set (resulting in crap job), completely change spells (cure -> dia? Lol.), or ham-handedly ignore the ability set entirely (e.g. level 30: transform into bear!).