littleadv's answer gives a concise summary of the system as it stands now, but much more changed than just the portion of the mandatory contribution that was diverted to the private plan. In broad terms, the balances of your accounts and your future benefit won't change. It's only the source of these benefits that's changing.
The Bloomberg article describes the changes this way:
The state will take over the amount of bonds that pension funds held as of end of Sept. 3 and turn them into pension liabilities in the state-run social security system... The state will assume control of 51.5 percent of pension-fund assets, including bonds guaranteed by the government and “other non-stock assets”
After the change, Polish workers that held bonds in the private portion of their retirement portfolios will instead have more payments from the state-run pension system. The balances of your retirement portfolio and your future benefits shouldn't change, but the reality may depend on how the state pension system is managed and any future changes the government implements.
The effect this change will have on future benefits isn't clear, because the change may simply delay the problem of high levels of outstanding sovereign debt, not solve it. The government stated that because increasing numbers of workers invested their money in private pension funds, less money went into the government's fund, which forced them to issue sovereign debt in order to cover the shortfall in their current pension liabilities.
The government's recent cancellation of government bonds in the hands of private pensions will decrease their overall outstanding debt, but in exchange, the government is increasing its future pension liabilities. Years down the road, the government may find that they need to issue more sovereign debt to cover the increased pension liabilities they're taking on today. In other words, they may find themselves back in the same situation years down the road, and it's difficult to predict what changes they might make at that time.