A bit late, but new to Shogun 2 and got some questions...

[Sar1n]

So I picked up S2 on Steam when it was free. Now I've actually started playing it...well, after looking around a bit with disappointment as vanilla without DLCs seems very bare, I've picked a Chosokabe campaign on hard difficulty. Seems to be a good starter.

Now first thing that bothers me. Is there a way to retrain older units to get the benefits from advanced buildings, like armorer? This is bothering me quite a lot.

For Nanban quarter, do I need just Christianity as majority religion in the province where I want to build it, or do I need to adopt it clanwide?

Edit: okay, adding a few more questions. I could use a second opinion on some units. I'm not sure if sword units in general are worth using in campaign. I can't spare a building for them in my main recruitment center-Satsuma-and they don't seem to have any advantage over naginata samurai that wouldn't be eliminated by a volley or two, since naginata samurai are much better armored. But I could be wrong, raw stats are one thing, field performance another...I've noticed that S2 seems to play "rock, paper, scissors" style harder than previous TW games.
Yari Ashigaru...they can really pull their weight in battles, but now I have more than enough money, so I was thinking of replacing them with naginata samurai who would be pulling all melee infantry duties. Main reason for this is AI fond of archer spam.

I've noticed a "Great Guard" cavalry being available in custom battle and according to ingame encyclopedia, it should be available to me as I have warhorses and both buildings built in my recruitment center...but cant recruit them.
And will another Black Ship ever spawn after I captured one? It's one helova killer, but I could use-and have money for-another.

And finally, how can I get more generals? Right now I have 3 elderly+daimyo, and I didn't get another offer for ages.

[Sar1n]

Hm, seems like I found most of my answers by myself.
Retraining-none. Nanban quarter-gotta change religion. Sword units-depends on style. Seems like I gotta pick either sword or spear for campaign and stick with it, no mixing. That said...tactics centered around fire rockets and katana charge seem to work damn well, better than traditional anvil+hammer with bows and spears.
Great guard are available only after becoming shogun. There can be only one Black Ship on the map. And do not neglect marriages...

[FootSoldier]

Dunno why noone replied to you but here I am 9 months later. I'm wondering how you got on with the game. Imho it's the best game design of the TW series. If any of your questions are still relevant I'm happy to answer, or if you have new ones. I've played the game to death several times, so hopefully could help.

To answer one question - how to get more generals?
Answer: Bribe them. Use your metsuke (get his levels up) and he can bribe rebel generals (usually occupying settlements gone rebel), and faction generals (much more expensive). If you want you can end up with 20 or 30.

Generals aren't as critical in S2 since stacks with captains don't go rebel.

[Krieglord]

" Imho it's the best game design of the TW series."

It still seems like such a lost opportunity, that they got the engine to work so well for Fall Of The Samurai, compared to ETW/NTW gunplay, but then never did more with it