Poster Presentations: Is it acceptable to present an update to a currently ongoing project as a poster at a conference? The project has been going on for about 5 years, but we have made large strides in the past year. We already presented the project last year at the same conference, but would it be acceptable to present the major updates at this year's same conference?

Thank you all.
Along with the commenter, I agree that it will depend on the field and the conference.  

However, in all likelihood there is a way to make it work, if you are just super interested in doing the trip. Consider to position the presentation clearly in the title and to carefully construct your abstract.  You want to clearly show the connection to the earlier work, but that things have changed also.  Use your judgment on how to do that.  "Progress" is likely too weak, but "Breakthrough" too gushy. In other words sell the talk (without being too blatant but still...don't hide your light under a bushel.) 

Probably, there are also some pitfalls to avoid.  Although I think the downside risk is not so huge.  But try to avoid losing a follow-on publication if that is something you care about.

Of course, you need to understand the conference if they decide not to have you.  Even if you got a breakthrough or unexpected results, they have a reasonable argument that the audience has at least seen the scope of the research problem and initial approach already.  Depending on their own supply/demand for participants will impact if they want to have you again.  

Personally I would opt more to go somewhere new and get some new people to see your work (from your own interests, not out of altruism).  It is also nice to use travel to see new cities. (I'm sure some will scold me for having boondoggle as even a small fractional component of academic travel, but researchers are not Biblical cows with bound mouths...getting an occasional fun trip to go along with hard work in the lab is not unreasonable.  Life is short.)