This will preserve pagerank, but not immediately. This means for a while (1 week-3 months), many of your pages will be dropped from their current rankings.
They will be returned, eventually. But Google has to update their entire index before you'll start to see any improvements.
When you're telling friends about sites "I found this great site, its called facebook" - for those people not in the know, the natural thing people do is to use the .com extension.
It's also why the .com domains command a higher premium on the domain name aftermarket.
As for .com's having more credibility - I reckon they have a bit more, plus you'll lose some traffic when they mistake the .net/.com. There's a few sites I visit that are .net's, and when I'm not on my own computer with browser history, sometimes I accidentally go to the .com. We paid around $1500 for the .com in... 2004 maybe? This made a ton of sense, because with our margins/gross/net, it would have made a ton of sense for a competitor to buy the .com - they'd have been paid back on it if they could've got it to top 5 in google for us. (To all the people who might say that we could then sue them, the answer is that you're 100% technically right, but that's ugly and you don't want to waste time/lawyer fees in court except as a last resort). Anyway, even if it increased our conversions or preserved lost traffic by a small amount, it would have been a good business move... ignoring the loss of search rankings, which wasn't planned on and hurt.
I suppose I had to learn that lesson sooner or later, so I'll go glass-half-full and be grateful that it was on tens of thousands early in life, instead of more later. But that was a pretty expensive lesson.