The nuke-from-orbit option:
You might consider switching to one of the RANS crank-forward bikes. http://www.ransbikes.com/
It is not a bike you are likely to win any races on and they have a few unique issues of their own, but it still looks fairly normal, and is more comfortable to ride than a regular bike--they use a different style of seat entirely. Prices start at ~$1000 for a whole bike, or they also sell framesets (since you already have a newer donor-parts bike anyway).
There is a website/message board specific to these style of bikes: http://www.crankforward.com/
(other relaxed-geometry uprights are discussed, there is no "RANS-bikes only discussion" rule... but the general opinion of people who've tried them all is that the RANS examples are the best)

You learn fast, little grasshopper.
However I am somewhat anti-upright-bike biased, having switched to recumbents a few years back. I had "normal" bikes for a long time and remember them well.
I don't have any now, but I did in the past.
No upright bike I've ever test-rode or owned was anywhere near as comfortable to ride as the recumbent bike I have now.

First off,,,,,,, if it is possible, you might want to take a medical detour, and investigate if anything is wrong with your tailbone (such as possibly a Pilonidal cyst forming). Even if that turns out to be true and you get that fixed, for a few months afterward you're still not going to want to ride any bike that puts pressure on that area.

Second--the saddle switch might work. You might pick out one cut-out saddle (with less padding than what you've got now) and try that. It might help, and is cheaper than $1K bike shopping.
I don't think that trying the smallest, hardest saddle is worthwhile, however--certainly not if you have to buy it to try it.... Some people around here have the unique problem of confusing the terms "comfortable" and "tolerable".

Third.... Also the position you are in on the bike can make a big difference in how agonizing it is to ride. Having it properly sized and adjusted won't make it perfectly comfortable, but will reduce the pain a lot. If possible, you might post a couple pics--side shot of the bike, and side shot of you on the bike in your normal riding position (lean against a wall).

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On the other hand, get a recumbent bike. Most recumbents only have one seat available, and it fits 99%+ of the human population just fine. (also no more hand or neck pain either...)
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