I *think* you could use the existing 16MHz bootloader with an internal oscillator (just by changing the fuses) if you tell boards.txt that your download speed is half of what you'd otherwise expect (57600 instead of 115200)
That worked for a poster yesterday on his 644p chip when he converted from external 16Mhz @ 5vdc to 8Mhz @ 3.3vdc in a breadboard setup. He just changed two entries in his boards.txt entry for the 644p board:
xxx.upload.speed=xxxxxx was changed to 1/2 the prior entry baud rate
xxx.build.f_cpu=16000000L was changed to xxx.build.f_cpu=8000000L
He didn't have to change any fuses as he was still using an external crystal.