Maybe your backup weaker wheel isn't so weak after all if it's rideable and your strong wheel is falling shabby!
Changing the cassette is very easy! 2 or 3 minute job!
Chainwhip to hold cogs in place while turning cog tool($7)...some make them out of an old chain.
Cog tool ($10?)
Wrench to hold/turn cog tool on cogs for removal.
Slide it off and onto the new wheel.
BTW, sometimes a few days of before a big event is beneficial!