I'll try to answer a question or two as I've had every set-up that you describe. Hopefully I'm remembering this correctly, but I was only about 15 when I had these bikes, and I still have my J-disc if anyone is interested:

1) You will not lose index shifting, or you shouldn't as long as the number of speeds on the shifters match up with the number of cogs. I believe that 6 and 7 speed both used the same rear derailler, but 8 speed required a longer throw for the derailler. If you have cables and housing and are somewhat mechanically inclined, I'd try it yourself or under the watchful eyes of someone who has done that before.

2) Both 6 and 7 speed used 126mm dropout spacing, so the cog spacing is narrower for the 7 speed (I think). If you have 6 speed shifters, they will only index with the 6 speed cogs. I think that 7 speed will only index with the 7 speed cassette. They are compatible either way if you stick with friction. If the 6 speed is a freewheel, then you'll get an extra cog or tighter ratio by switching to the 7 speed. If it's a cassette, then the 7 speed freewheel will not be of any real use.

Will you have to change anything else? I don't think so, but I'd switch to round chainrings.

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Brandon Marsh - Website | @BrandonMarshTX | RokaSports | 1stEndurance | ATC Bikeshop |

O.k. so at worst I will have to change over to friction shifting with the 7spd freewheel.

All the cogs I have are screw-on freewheels, not cassettes. The bike has 105 downtube shifters, hence the change to the Swift Shift.

Last, why go away from BioPace, I have access to DuraAce Cranks and DA round chainrings from the approx the same era, would I need to swap out my BB?