There was a change-over in the 50 satangs in 2008. The old style was in brass (issued since 1988). The new style is, I assume, copper-clad steel. This features an up-dated portrait of the king and yes there is a slight change to the style of lettering.
The same has happened to the 25 satang coin but as far as I know the brass one finished in 2007.
The new updated portrait of the king now appears on the 1 and 5 baht as well but I've not got any circulation 10 baht coins for this year yet so I don't know if they are due to change.
I think there is only one mint that produces the coins in Thailand.