I went ahead and installed 8.1 again, waited for the entire msgs to sync then upgraded. Whilst i was in 8.1 I didn't get the msgs I sent when using 10, after upgrading I had both pre 10 and post 10 msgs again.
I think what is happening here is that the new messaging service uses an entirely different sync method/sever, so the first time I downgraded to 8.1 before going to a newer 10 version, only some of the messages had downloaded in time thus when I got into 10 only those were present. After doing a hard reset those same ones came back but none of the missing ones.
So basically any messages that are on your phone when upgrading from 8.1 get synced into the newer 10 sync but it can't pull them in automatically from 8.1 sync. I've done a hard reset now on 10 and all messages come back in.
My feedback on this method :
a) WP8.1 got all my old messages very quickly,
b) I've upgraded to 10581, without hard reseting. New messages received since 10572 in my SIM2 (535) were synced as well
c) However, new SMS sent and received today (640, running 10549) were synced only with my WP8.1 (1020)
So my conclusion :
as long as your main SIM is in a build < 10572 (inc. WP8.1), the sync on newer builds will be incomplete. I suspect that if I upgrade my 640 to 10581, it would start syncing new messages with the 535 as well (but would stop syncing with my 1020 until it is upgraded to W10M, in the fullness of time).
Now what I have to do is :
a) hard reset my 535 and restore : I'll see which SMS are restored. If all of them, as of today, are restored, it would confirm the theory that after an upgrade to a newer build, old SMS are converted into the new standard,
b) if this is true, then it means that I'll refrain from hard resetting after upgrading my 640, to let it sync the new messages (sent from 640, since my upgrading the 535 to 10581 yesterday, I hope everyone is following), to be sure that all my SMS's are fetchable by build 10572 and above.
I agree, we shouldn't have to do all this. I'm also aware that my issue comes from the fact that I'm toying 3 Lumia, running 3 different OS/builds, including a dual sim device ^_^