IIRC the EXTJs version used by CQ is redefined under the namespace CQ.EXT. So it should be safe for you to include the version you want of EXTJS and use it where you need.
I never did it though so give a careful test first.
Thanks for replying. What i can make out from your comment above is, we can include any version of extjs like <Script src="/extjs.verxyz.js"></script> in components or pages and use it. But i am not sure if after includeing the js I would be able to use it CQ.ext... way.
Also is there a way to update the default extJs in cq5 with newer version. Some small example would really help alot.