I know my bike's rear cassette needs replacing, but before I do that, is it feasible to dismantle the 8 sprockets to reverse their spin? Can you increase the lifespan of a cassette by letting the cogs wear and tear the other way?
Assuming you have Shimano-compatiple Hyperglide cogs: No, you can't. The cassette body is not symmetric, the cassette fits in only one position. You could resort to adapt the cogs with a file, though. But shifting will be problematic at least, the cogs have certain indents to make shifting smoother.
The problem is that any rear cluster designed for indexed shifting has directional cogs. There are ramps embossed on the sides of the cogs to catch the chain pins and lever the chain up to the next larger cog when the chain is shifted. If you somehow reverse the cogs these ramps will be on the wrong side of the cogs and will be running the wrong direction.
Yes, this is possible,I have done this on a modern 9-speed cassette. You do have to knock the pins out of the cassette which hold it together and file down the small index slot on each Cog to reverse the Cog on the cassette and it will work. This takes me about 1 hour. I do this to my new cassettes so I am ready to flip the cogs when needed.You will need a new chain to match the new surface of the teeth on the reverse cogs. The shifting ramps do not matter, I do not even notice the difference. Since you are knocking the pins out of the cassette which hold it together the torque from pedaling goes on a single cog. It is a good idea to have a steel hub body and really tightened down the tightening ring on the cassette. In doing this I have no wear marks on my Hub body. I am able to sufficiently tighten and loosen the cassette with my little touring travel tool. No need to carry a chain wrench and cassette removal tool. It is also possible to reverse the chain Rings up front too. Go with all Steel chainrings. Get rings which are not recessed on one side for the screws that hold them together, then when you flip them the spacing between the chain Rings will not change . You can get steel chainrings from China for $4 each,the quality of the steel is good.I have found I can get roughly 6,000 to 6500 miles on a drivetrain before the chain starts slipping. By using two chains and swapping them in and out every Thousand Miles I can get two thousand more miles out of the drivetrain thus bringing the drivetrain up to 8000, 8500 miles. By using three chains I can bring the drivetrain up to around 10,000 miles.Then flip all the cogs and chain rings and start over. Another strategy is to take a cassette apart and get rid of the spacers then only carry the cogs. You do not even need all of them since you typically just wear out three or four. This is my drivetrain strategy for touring.