Do all the radiators heat up ? is there a reason why you're changin the pipes - or just because you have heard they don't last long ?
If the pipework is looked after, for example flushed when installed or any work carried out to the system. And inhibitors are added everytime the system is drained - then pipes will last as long as 15mm pipe.
The problems you get with 8mm and 10mm piping is that plumbers/heating engineers don't flush them when new or when connecting a new boiler to it, people drain their systems to move and remove rediators then they refill with water - no flushing and no inhibitors.
I still install 10mm plastic coated pipe, when required or requested. On new installations there's not the problems with sludge etc if looked after - and with pressurised systems you don't tend to get blockages.
So it's up to you, if it's working and the pipework looks ok - then leave it. If it's looking old and you're planning on fitting a new boiler or changing to a pressurised system, renew the pipework but it doesn't necessarily have to be 15mm.
Some people will disagree with using 8mm or 10mm completely for piping central heating systems, probably people that don't flush systems or add inhibitors or that has just had experience of sludged up pipes because they haven't been looked after. I have installed many systems like this and have never had any comeback or problems.