I don't think you can radically change the system in a short period of time, but you can leave state pensions as a minimum and encourage people to save for their pensions on their own.
It will be impossible! I said it, although I always say nothing is impossible but this one...yeah, it is!
The change could theoretically happen in Norway, theoretically, because they have a low population and one huge private pension fund from all the oil, for the rest of the countries it's a simple no because, there is no money!!!
In order to get the money for the ones at 70+ now you need to tax the ones at 40s working, but this system would mean you stop taxing the ones working now and you put all their money in a 401k model so, where is the money for the ones in retirement going to come from?
Best case scenario, best overall economic environment, political consensus (impossible with all the socialists) and this would need at least 4 steps of slowly building it by one quarter of contributions spread over a decade, so no way we're going to see in our lifetime.
Of course there is a way, so complete financial meltdown, nothing is left, nuclear war, we go back to stone barter method, and you can start from that immediately but...again, we most probably won't be alive to see it happening.