Single->Multiple entry visa change possible?

Hi there, does anyone know whether it's possible to change a single entry B3 visa to multiple entry?

I've been waiting forever for my work permit to get processed, but even if that comes through by the end of the month, I don't think I can get my ARC before the holidays. To make matters worse, I am running out of pages in my passport as well, so wasting two more (Xmas + Tet) on single entry visas is not an option.

Any advice appreciated.

Yeah, you can get a multiple entry visa with a little more money!

Yes, more money usually does the trick, but in my experience, a Cambodia run was necessary to get a multiple-entry on the next renewal. Of course, that was a couple years ago, and things change DAILY here. I'm hearing now (from the visa agents I know) that ANY kind of VN visa is getting more and more difficult to obtain. They are not happy about it, as it hurts their business.