Short answer:
Not allowed by the rules.
Long Answer:
As a DM, this isn't answered as a simple yes/no. There are, for me, things to consider:
First, is there a newly released paragon path/epic destiny which is more appropriate/interesting/desired?
I don't want to punish a player just because he's been with me from the start of 4E. For example, our ranger picked Stormwarden, but now that he has seen Lone Wolf, both the player and I agree that paragon path fits the character's behavior and attitude much, much better.
I would be much less likely to allow someone to rework a PP/ED that has been available the whole darn time.
Second, is the desired paragon path/epic destiny better able to represent how the character plays/appears/behaves?
Even if the player has had the opportunity all along, it is possible to realize that you've made a wrong choice. If someone picked, say, Pitfighter at level 11, but in the intervening levels realized that their character was becoming more meditative (and wanted to go Kensai) or more defensive (and wanted to go Iron Vanguard), I would be more likely to allow it.
If someone just wanted to change how their character worked, and not use the rules to better represent their character, I would be much less inclined to allow it, though I would if I felt that the initial choice was really poor and the character wasn't really reaping any benefits from their paragon path.
Finally, can the change in the character be explained "in the game"?
I am totally not a fan of "My character was an elf, but I want to play a shifter instead" syndrome. Some groups don't mind it, but it drives me nuts. If we can come up with a reason/explanation/excuse to allow a character to rework a Paragon path, then I am MUCH more likely to allow it, and I am extremely unlikely to allow a PP change without such a rationale, no matter how well-considered the change is.
For example, our ranger, who has worshipped Melora, is realizing that his destiny is to serve the Raven Queen. The player is OK with this, but the character is still having grave doubts about it. However, he is coming around to realize that the events of his life have led him to this point, and that no matter how much he loves trees and squirrels and flowers and birds and crap, he is someone that has to stop Orcus from killing the Raven Queen. The character is having a major life-altering event (and the character is also taking a divine multiclass feat to represent this), so I am OK with the character's PP changing. The ranger is realizing that "who he needs to be" is not "who he wants to be".
Other options would include resurrection, near-death experiences, religious conversions, solo journeys and the like. Perhaps in the down-time between adventures, one character goes on a voyage for several months (which pass uneventfully). He could return, much changed, while the same changes would seem weird between two subsequent encounters.
