There's really no 'safe' or 'unsafe'. Going in a car at all is always going to be more unsafe than sitting at home on the sofa and rear facing is always going to be safer than forward facing (the same goes for adults, you're better protected in an accident if you're rear facing, although that wouldn't work so well for the driver!). It's about your assessment of how low you need the increased risk to be before switching.
We will be keeping the kids rear facing until there are no more seats we can afford that will accommodate them rear facing, because any increased risk isn't worth it to me. I would go without luxuries in the grocery shop to afford it if we had to and I would stop every 45 minutes on longer journeys if the kids needed to stretch their legs. If my kids grow according to their charts, they should be RF until 6+, although I'm not sure about the heights of the seats available, they will certainly accommodate the weights though.
We will be keeping the kids rear facing until there are no more seats we can afford that will accommodate them rear facing, because any increased risk isn't worth it to me. I would go without luxuries in the grocery shop to afford it if we had to and I would stop every 45 minutes on longer journeys if the kids needed to stretch their legs. If my kids grow according to their charts, they should be RF until 6+, although I'm not sure about the heights of the seats available, they will certainly accommodate the weights though.