Glad Rag,
As reported last Oct, publicly, the fix is a set of engine "burn-ins" designed to entrench the Integrally Bladed Rotor (IBR) before the aircraft is stepped up towards its design limits through various Mods toward the end of SDD.
As far as I know, the cost is minimal if you don't include the cost incurred by schedule delays to the test program. I also don't believe they've added any weight either. Pratt have clearly taken a lesson from their own engine here, probably because it is at the cutting edge of tech and was not predicted.
I see this as a very straightforward, cautious crawl-walk-run back to the desired limits as the potential loss of an F-35 incurred by missing out the crawl-walk stage would be tragic and preventable.
That's what I know.