diff --git a/Cargo.lock b/Cargo.lock
index 830157efd38af..e9b609536b29e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4588,6 +4588,7 @@ dependencies = [
  "sp-io",
  "sp-runtime",
  "sp-std",
+ "sp-storage",
 ]
 
 [[package]]
diff --git a/frame/support/src/storage/migration.rs b/frame/support/src/storage/migration.rs
index 8e6beefa8886e..264c3c644e144 100644
--- a/frame/support/src/storage/migration.rs
+++ b/frame/support/src/storage/migration.rs
@@ -159,7 +159,7 @@ pub fn get_storage_value<T: Decode + Sized>(module: &[u8], item: &[u8], hash: &[
 	frame_support::storage::unhashed::get::<T>(&key)
 }
 
-/// Get a particular value in storage by the `module`, the map's `item` name and the key `hash`.
+/// Take a particular value in storage by the `module`, the map's `item` name and the key `hash`.
 pub fn take_storage_value<T: Decode + Sized>(module: &[u8], item: &[u8], hash: &[u8]) -> Option<T> {
 	let mut key = vec![0u8; 32 + hash.len()];
 	key[0..16].copy_from_slice(&Twox128::hash(module));
diff --git a/frame/transaction-payment/Cargo.toml b/frame/transaction-payment/Cargo.toml
index e6720e4b54ff4..a969f30086884 100644
--- a/frame/transaction-payment/Cargo.toml
+++ b/frame/transaction-payment/Cargo.toml
@@ -23,6 +23,7 @@ pallet-transaction-payment-rpc-runtime-api = { version = "2.0.0-dev", default-fe
 sp-io = { version = "2.0.0-dev", path = "../../primitives/io" }
 sp-core = { version = "2.0.0-dev", path = "../../primitives/core" }
 pallet-balances = { version = "2.0.0-dev", path = "../balances" }
+sp-storage = { version = "2.0.0-dev", path = "../../primitives/storage" }
 
 [features]
 default = ["std"]
diff --git a/frame/transaction-payment/src/lib.rs b/frame/transaction-payment/src/lib.rs
index 8e3a9e398d66a..bf55885363f08 100644
--- a/frame/transaction-payment/src/lib.rs
+++ b/frame/transaction-payment/src/lib.rs
@@ -99,6 +99,20 @@ decl_module! {
 				*fm = T::FeeMultiplierUpdate::convert(*fm)
 			});
 		}
+
+		fn on_runtime_upgrade() -> Weight {
+			// TODO: Remove this code after on-chain upgrade from u32 to u64 weights
+			use sp_runtime::Fixed64;
+			use frame_support::migration::take_storage_value;
+			if let Some(old_next_fee_multiplier) = take_storage_value::<Fixed64>(b"TransactionPayment", b"NextFeeMultiplier", &[]) {
+				let raw_multiplier = old_next_fee_multiplier.into_inner() as i128;
+				// Fixed64 used 10^9 precision, where Fixed128 uses 10^18, so we need to add 9 zeros.
+				let new_raw_multiplier: i128 = raw_multiplier.saturating_mul(1_000_000_000);
+				let new_next_fee_multiplier: Fixed128 = Fixed128::from_parts(new_raw_multiplier);
+				NextFeeMultiplier::put(new_next_fee_multiplier);
+			}
+			0
+		}
 	}
 }
 
@@ -792,4 +806,38 @@ mod tests {
 			assert_eq!(Balances::free_balance(2), 200 - 5 - 10 - 100 - 5);
 		});
 	}
+
+	// TODO Remove after u32 to u64 weights upgrade
+	#[test]
+	fn upgrade_to_fixed128_works() {
+		// TODO You can remove this from dev-dependencies after removing this test
+		use sp_storage::Storage;
+		use sp_runtime::Fixed64;
+		use frame_support::storage::generator::StorageValue;
+		use frame_support::traits::OnRuntimeUpgrade;
+		use core::num::NonZeroI128;
+
+		let mut s = Storage::default();
+
+		let original_multiplier = Fixed64::from_rational(1, 2);
+
+		let data = vec![
+			(
+				NextFeeMultiplier::storage_value_final_key().to_vec(),
+				original_multiplier.encode().to_vec()
+			),
+		];
+
+		s.top = data.into_iter().collect();
+
+		sp_io::TestExternalities::new(s).execute_with(|| {
+			let old_value = NextFeeMultiplier::get();
+			assert!(old_value != Fixed128::from_rational(1, NonZeroI128::new(2).unwrap()));
+
+			// Convert Fixed64(.5) to Fixed128(.5)
+			TransactionPayment::on_runtime_upgrade();
+			let new_value = NextFeeMultiplier::get();
+			assert_eq!(new_value, Fixed128::from_rational(1, NonZeroI128::new(2).unwrap()));
+		});
+	}
 }
diff --git a/primitives/arithmetic/src/fixed64.rs b/primitives/arithmetic/src/fixed64.rs
index 6f7e5fe84249f..af4dbf34e293b 100644
--- a/primitives/arithmetic/src/fixed64.rs
+++ b/primitives/arithmetic/src/fixed64.rs
@@ -49,9 +49,6 @@ impl Fixed64 {
 	}
 
 	/// Consume self and return the inner value.
-	///
-	/// This should only be used for testing.
-	#[cfg(any(feature = "std", test))]
 	pub fn into_inner(self) -> i64 { self.0 }
 
 	/// Raw constructor. Equal to `parts / 1_000_000_000`.