Depends on the severity of the nerf I guess. You should always keep within Tier A or Tier S ( Tier S being the highest ) for your champion pool. Unless...
The exception to this rule is when you've solely devoted all your time to a single champion. Regardless even if your champion was brought down to Tier B, and you're still performing adequately there's no reason to swap just yet. But lets say your win rate has suddenly went from 54 - 60% ( over 100+ games ) down to under 50%. It's time to rethink what you need to do to reach over 50% again. Reason being is, 50% is still climbing rate, but barely, and not always.
Always look at stats. NA.OP.GG is a helpful website for that.
There have been times where champions have been taken from Tier S and brought down to the lowest Tiers possible in 1 or over a series of patches.
There are many high elo " one-trick ponies " and you're able to see how multiple patch changes indirectly or directly affect their champions play a role in their ranked ladder success. Regardless, they're usually able to manage the same ladder ranking depending on how much game theory they accumulated in contrast to just champion mastery.
A good way to keep tabs on what champions are strong, are to look at the current standings for each role.
ADC
Top
Mid
Jungle
Support
You can keep an extra eye on the roles you play.
With every patch that comes out there might be, item changes, champion changes, mastery changes ( ETC ) will play a direct or INDIRECT role in affecting the Tier list for those respective lanes.
To briefly summarize how something could indirectly affect a champion...
Let's say the current Top 4 ADCs were.... ( not actual )
- Caitlyn
- Ezreal
- Tristana
- Vayne
Ezreal's build path is drastically different from that of Tristana and Caitlyn, who have almost identical items built.
For instance, Tristana and Caitlyn both build Infinity Edge as their CORE item.
But Ezreal never opts for this option and his core item happens to be Tri-Force in this case for the #2 spot in ADC ( Certain builds on champions could dictate their position on the ladder )
So Infinity Edge for #3 Tristana and #1 Caitlyn.
Tri-Force for #2 Ezreal.
Let's say Infinity Edge gets a significant enough buff to make the current ADCs who builds this item to rise in the Tier list.
Ezreal now indirectly gets affected, even though no changes were of any relevance to him ( on the surface )
Tristana gains new strength and gets bumped to #2. Now Ezreal drops a rank since he gets out classed by the champions below him who were affected by the item change.
This applies to champion changes, item changes, mastery changes, and shifting meta picks. If suddenly a new champion came into the lime light that countered Ezreal, but not Tristana, that might change things up a bit.
Same thing with how not every ADC utilizes the same mastery keystone ( although most of them do )
It's the same idea throughout.