While the new Cube Client can be used with both First-Gen Cubes and Next-Gen Cubes, and some Next-Gen features might safely be experimented with on First-Gen Cubes, using PLA is NOT one of them.
I am NOT planning, under any circumstances, to test printing PLA in my First-Gen Cube until someone at 3D Systems confirms that it is OK. And, I do not think that is likely.
Frankly, I can't think of any good reason for doing so. ABS is stronger than PLA and we have a heated bed.
No one at 3D Systems has confirmed my theory; but, my experience with
PLA in a RepRap machine tells me that not only will PLA not work well at the First-Cube's temperatures; but, may damage the feed mechanism. That potential just does not seem worth the risk to me, for so little gain.
Printing temperature for ABS is around 240C to 248C. Printing temperature for PLA is begins at 190C and typically averages around 210C. That's quite a big differential.
Here's my theory of the probable outcome...
First, remember that the filament is being pushed down a shaft before being melted at the tip. In order to work properly the filament must be solid at the entry of the shaft to be heated and only reach the right temperature at or near the tip. With ABS, the shaft at the entry point is just slightly lower than the melting point of the ABS itself. So, the feed gear pushing on the filament is able to push the filament some distance into the shaft before it reaches the melting temperature at the tip.
Since the optimal melting temperature for printing ABS is quite a bit higher
than the optimal temperature for PLA, running PLA through the head of
the First-Gen Cube is likely to heat the PLA to the melting point higher up in the shaft where the
melted plastic could back up into the threading mechanism.
So while the entry to the shaft might be below the melting point of ABS it is likely ABOVE the melting point of PLA. The PLA material will be too molten to be able to be pushed deeply into the shaft and I expect the molten PLA will flush back into the housing and then cool, causing more than a simple clog. It could cause actual damage to the head feeding mechanism.
I'm aware that I may be completely off base. But, I'm not willing to risk it. I'll stick with the ABS for which the First-Gen Cube was designed.