Writing on behalf of a colleague - he is Delta Diamond who currently has a US based account, but has moved and is now a foreign resident.

As someone who regularly travels in business class, does it make sense for him to shift his account out of the US as a foreign based Delta FF? I suppose this eliminates the MQD requirement, but it seems to also result in lower mileage rewards for his travels as opposed to being a US based FF.

Also, given his location, future travel will mainly be on KLM metal.

DL passengers earn the same number of regular MQMs and regular RDMs regardless of whether the address on the FF account is USA or Europe.

There are some promos based on the address, such as all of the bonus miles for those based around SEA and NYC.

Whether he should move to a different FF program will depend entirely on what his new flight schedule looks like and where he actually moved to.

If he's in AMS and regularly has to fly back to the states, then either staying with SkyMiles or moving to Flying Blue makes sense. If he's now in Germany and flies mostly to Asia or vice versa then it doesn't make sense to say with DL or any US FFP.

A little more detail will yield better advice

Apologies, fair enough, I should have provided more information.

He is based in Lima, Peru. For travel to Europe he usually travels KLM metal, given that it is the best connection for us. He is an American so he regularly travels to the US for business and family visits, and in that case tries to fly Delta.

His question is really about whether there are major differences transferring his Delta Skymiles account from US-based to Peru-based.

In that case, SkyMiles would probably be the best choice. Assuming that he requalifies for Diamond, those GUCs are extremely valuable, and there is no similar offering with Flying Blue. SkyTeam has a better presence in LIM than the other alliances for TATL flights. BA is just now adding a flight to LGW, and Star Alliance doesn't having anything to their European hubs.

If most of his travel is going to be to Europe or the US, then staying with DL isn't a bad idea, as he can get a lot of places in the US/Europe with one stop in ATL or AMS, There is also a couple time a week AF flight to CDG.

I don't personally know much about Peru based FF programs available, however if this move is temporary (couple of years or so), I'd stick with DL unless he finds himself needing to do a lot of flights within Latin America.

The SkyTeam partners in Latin America are AR and AM, neither of which would make sense for Peru.

To me, the only possible argument for switching would be to change to FB if the KLM flights earn a lot more status and redeemable miles that way, which I suspect is unlikely. The cost would be loss of upgrades (GUCs/RUCs as well as free elite upgrades or upgrading with miles), which could be huge.

BTW, is Lima considered northern or southern South America for upgrades? Is there D1 service on the Lima routes?

The colleague should familiarize himself with the rules about purchasing upgrades with miles or money on KLM flights and on DL flights from AMS. The good recent news is that DL GUCs can now be used to upgrade on KLM worldwide, apparently on any non award revenue fare class.

OP is not asking about whether the friend should switch to another loyalty program. The question is simply whether the friend should update his address on his DL SkyMiles account.

No, there should be no major differences aside from eliminating the MQD requirement.

Sometimes Delta does run special promotions targeting flyers who reside in competitive markets (e.g., NY, WA, AK, CA), but that would not be enough reason for me to keep my address in the US -- especially if my address was not in one of those markets to begin with.

I'd keep DL and update address to get rid of MQD requirement.

As for award mileage, this has to do with the point of departure. Ex-JNB to US fares are a quarter what ex-US to JNB are. It's a matter of pricing be based on the local economy. Same thing happens with mileage awards nowadays. It's a lower amount of miles ex-Europe then ex-US. However note that fares from non-US locations can have high carrier fees involved in them.

Also, fares originating outside of the USA can have very different fare rules. For example, on many discount fares, the entire ticket is lost if changes are needed versus facing a change fee of $200-500.

A more descriptive subject line wouldn't hurt either...

Thanks to all for their comments. This has been very helpful.