l have a 2005 yz250f and l wanna put 2008 plastics on it with new graphics, the bike is brand new but l am not sure if l can modify the new plastics on a 2005, can it be done?
But is it possible to bolt it on

whatever you want man

Check with your local shop. See if the part numbers cross match somehow.

Next best thing is to take off your 2005 plastic and take it to the shop to see how it lines up with a 2008.

No, not without some serious modifications, still not sure it is really even possible though. Stick to the plastics made for your bike. You will find that they fit much better.

You should just be able to buy new plastics that will fit the model you have if you just contact a local yamaha dealer. Then pick whatever graphics you like and get them made for that model and put them on. l think you will find that the plastic shape changed from 2006 to 2007 as most graphics places sell 2003 to 2007 as one size and then 2007 to 2008 as another.

No the 2008 and 2005 models use different plastics.