There are many reasons why LEGO® parts get redesigned, for example to strengthen a part or to improve the moulding process. But sometimes a whole 'family' of parts will get redesigned, and this process can take years for
TLG to roll out because moulds need to reach the end of their working lives before the expense of replacement is undertaken.
Over the last five years or so, the family of
arched bricks have been altered and I was prompted to write this post because what I think/hope is the last of these, the 1X8X2 arch, has now begun to appear in its new form. The essential change to arched bricks is that they are being "raised" - the interior curve no longer sits flush to the bottom of the brick.