I have an accepted conference paper with a major error in it, should I ask for it to be changed? I have submitted a paper to a conference with a scientific error in it.

Essentially I have a concept and I have claimed it works in two conditions. Actually it only works in one of them. Technically, my experimental results are correct, but its only a small test and when extrapolated to full scale the error can be found and also actually predicted theoretically. The overall concept is still correct however for the other condition and I am still working on it in future research.

Should I ask for the paper to be changed? The conference isn't until 25th August but the submission deadline has passed.

I don't want a public document where people will repeat my experiments and find that some of what I have claimed is incorrect.
Yes!

Better to get it corrected than to be questioned/rejected later.