Let's take a current BFL Bitcoin ASIC for example. (5 GH/s, 25 GH/s, 50 GH/s, etc)
I know it's hard to think of ASICs as being outdated because they are the best mining technology out right now, but GPUs were once the best and they became obsolete by new technologies. And now those same GPUs are used to mine alt coins because it's more profitable.
It's inevitable that this will happen to the current generation of Bitcoin ASICs down the road, if only to be surpassed by far superior ASICs.
Would this conversion just require a software update? Or would there be firmware updating required?
Every ASIC on the market today will be a doorstop in 1 year. Better ASIC's will be made and a Bitcoin ASIC has no other purpose than mining SHA, also most alternative coins use script (except PPC, TRC)
The current generation have no other use and will become a doorstop, if someone says otherwise they are selling snake oil.
EDIT: This is why time to market is important, getting your ASIC 3 months earlier is all the difference between doorstop and ROI.