Hi, let me clarify things a little more: the motherboards of AV32R SP, AV32R DP and AV192R start as the very same bare boards, but are then differently populated (with electronics) in order to fit their final target platform. The same can be done afterwards 9i.e. from populated boards), hence you can convert between these products, but it's more cost-effective when done from the bare boards (as no unnecessary components need to be removed). This is the case for new boards.
Old motherboards require (partially) significant modification to get them to the upgraded stage, say when upgrading from SP->DP or AV192R. Electrically the result will be very similar between an upgraded and a new board, so an upgraded processor, say AV32R DP, will sound very much the same as an AV32R DP made from brand new parts.
The improvement of boards (over time) results in subtle sound quality improvements within a product, i.e. an AV32R SP from 2004 will sound better than an AV32R from 1999.
In case new motherboards were requested, the removed boards are scrapped, as there is really nothing one could do with these old generation boards.
There have been, if memory serves me well, 6 different motherboards since 1999.