its the exact same build, but if there's any particular service you need from your carrier that actually runs on your phone, it might get lost, or not, w10m will be slapped on top of whatever you had before, but everything is buggier so even if you manage to keep your apps, somehow they will crash where they never crashed before on 8.1
carrier wont know the difference, you will get their updates anyway, happened with my 928 and denim, a year after I got 8.1 through old insiders app(dev prev) they decided to release 8.1+denim but since I already had 8.1 it just installed denim on top, and all the Verizon bloatware
so here's how it will(most likely) go for you, you get on the production ring, build 10586.164 gets installed, everything you had remains there(doesn't mean it will 100% work, but most likely it will, just buggier) and then when your carrier finally releases something, you will get that too, unless you go to fast ring and get a build that doesn't officially exist yet, the carrier specific "bits" are just installed on top of whatever is the latest build said carrier released to you, so essentially if you wait, your carrier will send you 10586(update1) and THEN give you another update from them(update 2), so if you take a shortcut to step one(10586) it makes no difference later, you simply got the update 1 faster
btw I say "buggier" but I don't mean "unusable" just don't expect the near perfection level of 8.1's reliability and stability, if you're a windows fan or just hate the others too much to switch, this is it, 8.1 and all previous OSes will just fade away and die alone(literally) you will realize your phone is no longer "smart" if you stay on 8.1, sooner or later every service will pull out support or stop working altogether
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