Is it possible to withdraw a conference paper after the rebuttal period? Maybe it sounds like an odd question, but it's really important to me! 
Recently I received the reviewers' feedback as the rebuttal phase of the conference, and I sent them an answer letter regarding their comments.

As a matter of fact, two of the reviewers pointed out a theoretical problem regarding my proposed methodology which somehow disproves my claim. To be honest, I'm fine with that part as they helped me to understand what I was missing in my work. 
And, based on the reviewer's comments, I expect the paper to be rejected in the end with 80-90% likelihood.

The problem is that they will send the notifications to authors 1.5 months from now, But we already figured out the solution to fix the missing theoretical part and I also gathered the results supporting that. So now I have the modified version of that paper almost ready to submit, but I have to wait 1.5 months until I receive the official notification from this conference to be able to send the paper somewhere else otherwise it'd be against the regulations.

My question is: is it proper to ask the conference chairs to revoke the paper now, and then submit the paper somewhere else? Or would it cause bad impression or might damage our reputations?
I suggest that you write to the chair of the program committee, explaining that (1) you agree with the reviewers about what was missing from your paper, (2) you've revised the paper to correct the problem, (3) you realize (or at least believe) that it's too late to modify your submission to this conference, (4) you would therefore like to submit the the revised version elsewhere, and (5) you would therefore like to withdraw your submission to this conference.  I would expect a reasonable program chair to let you withdraw the paper; after all, the conference gains nothing by forcing you to wait for an official rejection.  

I'd also suggest mentioning that you're grateful to the reviewers for pointing out the problem.