I've got a bike with S.I.S. that I *may* want to change the crankset on. If I do change the crankset, will the fact that it's S.I.S,. limit me to Shimano cranksets? Because I was thinking Sugino, until I remembered that I'm dealing with S.I.S.....

Sissy shifting is what it used to be called.

Shimano would say it is a no go, what you need to measure is the distance between the ring centerlines, if the same happy. Do note that Shimano and Sugino may not use the same thickness of chainring material, so measuring the distance between the rings may not be a good guide.

If good, bench test and field test, on the early SIS shifter, the factory jump was 53/39, if greater than that, might be a problem, it 50t or under for the outer ring, may not work. Don't sell your old crank until the new works.

I'm assuming that if the spacing between the chainrings is the same between the two cranksets your SIS should work. Your derailleur will probably need adjustment.

edit: beat to the punch

If it is a double crankset it will make no difference at all.
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And to get rid of the S.I.S. altogether? Do I need to replace the entire drivetrain?

What is your ojective? STI, Ergo, Positron or friction... those are about the only other options I know of... well, internal hub gearing too...

I've got a bike equipped with Biopace. My mechanic is strongly urging me to put a round chainring on (I'm converting the bike from mountain bike to commuter) so I was thinking of a touring crankset. Only there's that S.I.S. issue.

I do like the rapid fire shifters for commuting ease, but I've never liked the "system' approach that you get locked into with indexed shifting. So I suppose one objective would be to go to a round chainring but keep S.I.S., and the other would be to go to friction, so I can use whatever components I want.

You should be able to just swap out the chainrings.
In fact, if you want to swap Biopace for round ones PM me. The egg-shaped rings are kind to my beat-up knees.
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If the objective is to eliminate biopace, then swap the chain rings as mentioned above...

If you like the rapid fire, then leave everything else alone.

As you indicate most indexed shifting is somewhat proprietary, even though some will cooperate.

Unless you have a burning desire to do friction, I see no reason to change systems to allow interoperability unless you have issues with what you have. To do otherwise is like trying to put a lawn mower engine on your motorcycle because you don't want to be locked into Honda... even though you would give up features.

The 6-speed 7400-series Dura Ace that I've got on my Rossin allows me to switch off the racheting function on the right lever (the left is straight friction). Don't have to change anything in the way of equipment.