In the 10 days following your upgrade, you may see your speeds vary. This is perfectly normal, as tests are carried out during this period to see what speed your line can support. It's important that you stay connected during this 10 day period so that these tests can take place. After 10 days you should see your speeds stabilise at the fastest speed your line can support.
So does this mean after the 10 day period, I can expect my router to sync directly to a stable speed. Meaning, at the moment I am within the 10 day window and so each time I restart the router in the morning it sync at 3.5Mb and stays there all day till nightime and the stable speed really is something like 3Mb.
So I am hoping after the 10 days period the modem will just boot and connect at 3Mb, there is no point in connecting at 3.5Mb, otherwise I just end up enduring the unreliable few days of synchronising steadily down to 3Mb. Why not just start at the point known to be stable?