1 - Piece of cake... Most of the PIC's in the same family are (i.e. 28 pin for 28 pin, 40 pin for 40 pin, and so on), 99% compatible at the pin level with a newer PIC having more functionality on the same pin or set of pins, about 95% compatible at the PBP level, and probably around 80% compatible at the assembly level. For instance, I built an MP3 player, started off with a 16F877A, moved to an 18F452, then an 18F4620. No change in hardware, and minor changes in the PBP source.

2 - The 74C922/74C923 aren't that easy to find anymore and they're a bit expensive. You could do the same thing with a much cheaper 18 pin PIC, and program it yourself to include things like shift keys, control keys and things like that. Can't do that with a 74C922...

3 - I don't own a dev board of any sort. There is nothing wrong with any of the boards sold by MeLabs, they're all good...however, I do believe you get a better deal with the ones from www.mikroe.com, the EasyPic4.
And I (and others will say the same thing) think one of the better programmers out there is the PICKit2, upgradeable, support direct from Microchip, it's USB, self powered, and so on. I just bought one, works great.

4 - PIC-AXE with a bootloader - all that means is that you can reprogram the chip without having a programmer, usually all you need is a serial port or something similar, but at the expense of a couple of pins and some code space. With time, you could make your own bootloader...or you could buy or download 'bootloader' code from various places on the 'net.

4a - and yes, it's similar to a JTAG port.