This has been suggested for machines like assemblers, cutters, stampers, etc.
I am suggesting that EVERYTHING with upgradeable speeds is given the ability to downgrade from it's current highest rate. The way things are as of now is any upgrades purchased instantly get applied worldwide, which while technically just increasing output without effecting ratios, effectively creates bottlenecks from upgraded buildings being TOO efficient.
This could be mitigated by just adding splitters to the lines off of your old buildings or lines, but in a game about space conservation, I'd like to be able to just stick to one building per completed setup(or however many buildings you can connect from your base setup).