You can't swap functionality around, however the UNO has 6 Analogue capable inputs, 16 Digital pins, of which 6 can be used as PWM outputs. The UART, I2C, TWI, and SPI pins are spread out, so you're not likely to run into pin clashes. I've not paid that much attention to the Atmega328 datasheet, but I think the UNO uses most of the chips capabilities.
The Mega on the other hand has a couple limitations that have caused me issues.. (why they only implemented 4 out of the possible 8 external interupts, and shared two of them with the TWI pins I really don't know)