What I could understand is that since the grammar of Perl 6 is written in Perl 6, One can override the grammar and change some of the syntax itself? Am I right if say this?

yes

What other possibilities can be achieved and upto what level is it available

It goes very far. There is basically nothing that you cannot change. By overriding the <EXPR> rule (or some other fairly top-level rule) you can substitute the complete grammar with your own, if you feel like.

However the question arises to which extend you can change the grammar without changing the compiler backend. I don't think that questions is fully answered yet.