Let your paladin stay holy mate, its alot stronger / more viable.
Retri state is decent/a bit underpowered.
They have a nice burst but lack strong defensives.
This has always been the case, but in the past this was compensated (in a few patches)
with OP healing, the problem is that in MOP they gave rets more utility (sac dispel etc) but
took away their 'OP' healing, making them very weak defensively.
Playing double dps as a ret forces you to play very very defensive and pump all holy power in healing,
greatly reducing your dmg etc... and if you face a decent/strong healer+dps comp this will never be enough anyway.
At low rating/mmr you can still perform well ofc, but above 1.9 or somethng im guessing you will just
get facerolled by all the healer/war-dk-feral-mage-hunter comps out there.
Another problem with the comp is the lack of cc, the best talent setup would still be fistofjustice(ret) and strangulate glyphed for the dk,
in the past this was enough to get a kill, you would just rush in and both pops cds on the healer, then get his trinket with stun/strang
and kill him with the other, back then rets could also do insane offheals without losing that much dmg, but ye this simply doesnt work anymore.
So no, dont go ret
Stay holy and let him take the fear talent, healing you should be no problem and he can support your uptime
with freedoms and cc your offtarget with fears/hammerofjustice/blind.
P.S. Based on DRs alone it would be better to take Repentance instead of fear (8s rep into 5s hoj into 6s blind, all on a different dr)
but since you are a dk I think its better to take fear unless you can really avoid getting your dots on the cc target (rep breaks on dots, fear doesnt)
Sorry for my bad english.