Authorship change on conference paper after conference program is printed

I am a coauthor on a psychology conference paper that was accepted based on a short abstract. The first author became unable to attend the conference and offered the opportunity to present to the coauthors. I accepted and later found out that I need to prepare the slides and text for the presentation.

The first author had presented similar material to a very different audience (a practitioner audience in a workshop setting in Spanish) and offered me the slides from that presentation. The material needs considerable reworking: it needs to be adapted for a researcher audience; have a theoretical framework integrated; be shortened considerably; and be written in (or translated to) English. I should also say that I made scientific contributions to the overall project and these particular results.

My colleague agreed that given the extent of the changes, I should be first author (up from fifth author where I am currently) and he second (remaining authors unchanged). He contacted the conference organizers with the request, but the conference program has already been finalized. The first author believes that this means that no authorship changes can be made, but I am not sure.

There will be no conference proceedings, so the conference program is the only documentation, other than the presentation slides and any documents we distribute.

Can authorship changes be made in the presented version of a conference paper even if the conference program is already finalized? Can we list the paper on our cvs differently from how it appears in the program? Are there any written guidelines on this topic?