diff --git a/sway-core/src/decl_engine/id.rs b/sway-core/src/decl_engine/id.rs
index cdbd0fccffd..915cb0f33bd 100644
--- a/sway-core/src/decl_engine/id.rs
+++ b/sway-core/src/decl_engine/id.rs
@@ -148,7 +148,7 @@ impl SubstTypes for DeclId<TyFunctionDecl> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -160,7 +160,7 @@ impl SubstTypes for DeclId<TyTraitDecl> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -172,7 +172,7 @@ impl SubstTypes for DeclId<TyTraitFn> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -184,7 +184,7 @@ impl SubstTypes for DeclId<TyImplSelfOrTrait> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -196,7 +196,7 @@ impl SubstTypes for DeclId<TyStructDecl> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -208,7 +208,7 @@ impl SubstTypes for DeclId<TyEnumDecl> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -220,7 +220,7 @@ impl SubstTypes for DeclId<TyTypeAliasDecl> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -233,7 +233,7 @@ impl SubstTypes for DeclId<TyTraitType> {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(self)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(*self, decl);
+            *self = *decl_engine.insert_modified(decl, *self).id();
             HasChanges::Yes
         } else {
             HasChanges::No
@@ -241,15 +241,6 @@ impl SubstTypes for DeclId<TyTraitType> {
     }
 }
 
-// This implementation deviates from all other DeclId<...> implementations.
-// For more, see https://github.com/FuelLabs/sway/pull/7440#discussion_r2428833840.
-// A better solution will be implemented in the future.
-//
-// TL;DR:
-// When a constant is declared inside a function, its value is shared by every
-// “version” of that function—that is, by all monomorphizations.  If we “replace” the
-// constant, as other implementations do, we would change its value in *every* version,
-// which is incorrect.
 impl SubstTypes for DeclId<TyConstantDecl> {
     fn subst_inner(&mut self, ctx: &SubstTypesContext) -> HasChanges {
         let decl_engine = ctx.engines.de();
diff --git a/sway-core/src/decl_engine/ref.rs b/sway-core/src/decl_engine/ref.rs
index 4832fe3b3ab..fb32dcdd6da 100644
--- a/sway-core/src/decl_engine/ref.rs
+++ b/sway-core/src/decl_engine/ref.rs
@@ -260,13 +260,14 @@ impl<I> Spanned for DeclRef<I> {
 impl<T> SubstTypes for DeclRef<DeclId<T>>
 where
     DeclEngine: DeclEngineIndex<T>,
-    T: Named + Spanned + SubstTypes + Clone,
+    DeclEngine: DeclEngineInsert<T>,
+    T: Named + Spanned + SubstTypes + Clone + TyDeclParsedType,
 {
     fn subst_inner(&mut self, ctx: &SubstTypesContext) -> HasChanges {
         let decl_engine = ctx.engines.de();
         let mut decl = (*decl_engine.get(&self.id)).clone();
         if decl.subst(ctx).has_changes() {
-            decl_engine.replace(self.id, decl);
+            *self = decl_engine.insert_modified(decl, self.id);
             HasChanges::Yes
         } else {
             HasChanges::No
diff --git a/sway-core/src/language/ty/expression/expression_variant.rs b/sway-core/src/language/ty/expression/expression_variant.rs
index 8931c9101b5..b684086a17f 100644
--- a/sway-core/src/language/ty/expression/expression_variant.rs
+++ b/sway-core/src/language/ty/expression/expression_variant.rs
@@ -936,7 +936,7 @@ impl ReplaceDecls for TyExpressionVariant {
                         .replace_decls(&inner_decl_mapping, handler, ctx)?
                         .has_changes()
                     {
-                        decl_engine.replace(*fn_ref.id(), method);
+                        *fn_ref = decl_engine.insert_modified(method, *fn_ref.id());
                         has_changes = HasChanges::Yes;
                     }