Fuji,
I find your example of changing TAFs quite common. A few weeks ago I went to Shoreham. The TAFs showed something like BKN012 but BECMG SCT020 before I got there, and continuing to improve all day. It was similar all over the S East, and I had no reason to doubt the TAFs. When I got there (less than 2 hours later) it was OVC007. According to the plate, the MSA for the 25nm circle round Shoreham is 2100.
Nuff said?