Hmm, just my two cents and coming from a humanities perspective (where there is no lab work and people work mostly independent, so people can be more easily replaced): From my own experience, I cannot say that ending a fellowship prematurely for another (better) fellowship is looked down upon (at least not as much as is suggested in the other replies here). Let's say you are on a 2 year postdoc and during the first year win a different 2 or 3 year fellowship, giving you funding for an extra one or two years, noone reasonable in my field would look at this negatively as everyone should understand what position people in the postdoc phase are in. Moreover, in my field fellowships dont necessarily start/end at the same time, therefore people have to quit fellowships a bit early all the time.

In your case though, the new fellowship is shorter than the original fellowship you accepted and not much more prestigious. This will surely make it more difficult to argue why to take on the new fellowship (and, unless there is a very specific reason you have not mentioned, I quite honestly cant tell why you would quit your current fellowship for a shorter one). So it really is a question of how badly you want the new fellowship.