Have you compared the cost of new hardware versus the cost of retraining staff in Erlang and re-architecting your software in a new language?
I wouldn't underestimate the expense of retraining yourself (or others) and the cost of hiring people conversant in Erlang (who are going to be a lot harder to find than Java people). Servers obviously cost in terms of their storage costs / power / maintenance etc., but they're still a lot cheaper than qualified staff. If you can make progress and remain scalable whilst using your current skillsets, I suspect that's the most pragmatic approach.
