Welcome to the KLM forum!

I try to answer your questions:

If I were to relocate, would it make sense to switch from WorldPerks to Flying Dutchman? Is it possible to convert existing WP miles to FD points, and are they devalued a lot in the conversion?

It makes sense if you find the KLM FD program more suitable. You can convert 1:1 mileage and status. But it can be a process of up to 8 or 10 weeks...

Which program is more restrictive when it comes to mileage redemption for u/g's and awards? E.g. NW allows WBC upgrades only from B to I and from Y to Z; is KL equally restrictive, or does it allow u/g's from more discount-coach fare classes? And are WBC awards easy (or at all possible) to redeem on KL?

Certainly NW is more restrictive. You can upgrade from any standard eco fare and you get miles on any published fare. Mileage earning is identical to NW WP also for elite bonus. As for awards KLM is great, as a GE or PE you'll get mostly any award if you are a little bit flexible and don't fly in the very peak weeks of the year. This is on KL metal only, as for NW and CO it's more tricky (comparable to your current situation at NW WP).

Also, what are some of the typical award levels, e.g. how many points for u/g or free r/t Europe/US or Europe/SEAsia in WBC? I tried to look for the latter info on the KL website but it seems that the award levels are posted in a secure area to be viewed only by existing FD members.

Awards: eco euro-US 50k, biz 80k, upg. 20k oneway. europe-far east 100k eco, 150k biz, 40k oneway upg.

Lastly, how is the KL presence in the London area, i.e. flight frequencies to the AMS hub from LHR, LGW etc.?

They have a great frequency on LHR-AMS (about 10 flights a day) and LCY-AMS (about 5 a day). Only tricky thing is to get a paper ticket there. AFAIK the only option is the office at LHR.

Hope I could help!