Loss Cubes are not distinguished between previously acquired and newly acquired. Any can be discarded, not just new ones.
One could argue that point. On page 12 of the new Mayfair version rules, it talks about taking loss cubes. After the basic discussion it says:
If you selected Sloan, discard half the loss cubes you take (rounding up).
At first glance it might seem "the loss cubes you take" means "newly acquired". However, the sentence immediately following reads: "E.g., if you had 9 loss cubes you would discard 5 of them, leaving you with 4." It's hard to imagine accumulating 9 loss cubes in one turn.
Furthermore, the descriptions of Sloan's benefits on pages 5 and 16 read, respectively: "Discard half, rounded up, of the loss cubes you have during phase 8..." and "During phase 8 discard half, rounded up, of the loss cubes you have..." (emphasis mine).
On balance, then, it appears that half of the total is the right call.