diff --git a/pkg/storage/imagestore/imagestore.go b/pkg/storage/imagestore/imagestore.go
index df8b55c4..a4d39c82 100644
--- a/pkg/storage/imagestore/imagestore.go
+++ b/pkg/storage/imagestore/imagestore.go
@@ -213,6 +213,80 @@ func (is *ImageStore) initRepo(name string) error {
 	return nil
 }
 
+// promoteBlobCandidate handles the core logic of copying/linking a single blob candidate
+// into the global blobstore and registering it in the cache.
+//
+// blobCandidate holds metadata for a blob selected from the pre-blobstore layout.
+type blobCandidate struct {
+	repoName string
+	blobPath string
+	size     int64
+}
+
+// repoBlobRef tracks a blob's presence in a specific repo (for cache registration).
+type repoBlobRef struct {
+	digest   godigest.Digest
+	repoName string
+	blobPath string
+}
+
+func (is *ImageStore) promoteBlobCandidate(
+	candidate blobCandidate,
+	digest godigest.Digest,
+	globalBlobPath string,
+) error {
+	// ensure algorithm dir exists in _blobstore
+	algoDir := path.Join(is.rootDir, storageConstants.GlobalBlobsRepo,
+		ispec.ImageBlobsDir, digest.Algorithm().String())
+	if err := is.storeDriver.EnsureDir(algoDir); err != nil {
+		is.log.Error().Err(err).Str("dir", algoDir).Msg("failed to create algorithm dir")
+
+		return err
+	}
+
+	if is.storeDriver.Name() == storageConstants.LocalStorageDriverName {
+		// local filesystem: use hard link (no extra disk space)
+		if err := is.storeDriver.Link(candidate.blobPath, globalBlobPath); err != nil {
+			is.log.Error().Err(err).Str("src", candidate.blobPath).Str("dst", globalBlobPath).
+				Msg("failed to link blob to global blobstore")
+
+			return err
+		}
+	} else {
+		// S3/GCS: copy the actual blob content
+		content, err := is.storeDriver.ReadFile(candidate.blobPath)
+		if err != nil {
+			is.log.Error().Err(err).Str("src", candidate.blobPath).
+				Msg("failed to read blob during upgrade")
+
+			return err
+		}
+
+		if _, err := is.storeDriver.WriteFile(globalBlobPath, content); err != nil {
+			is.log.Error().Err(err).Str("dst", globalBlobPath).
+				Msg("failed to write blob to global blobstore")
+
+			return err
+		}
+	}
+
+	// register global blobstore path as the master/original cache entry first,
+	// so that subsequent PutBlob calls for per-repo paths go into DuplicatesBucket
+	if is.cache != nil {
+		if err := is.cache.PutBlob(digest, globalBlobPath); err != nil {
+			is.log.Error().Err(err).Str("digest", digest.String()).
+				Msg("failed to update cache with global blobstore path during upgrade")
+
+			return err
+		}
+	}
+
+	is.log.Info().Str("digest", digest.String()).Str("repo", candidate.repoName).
+		Msg("upgraded blob to global blobstore")
+
+	return nil
+}
+
 // upgradeToGlobalBlobstore migrates blobs from per-repo directories into the global _blobstore
 // for older zot releases that did not have a centralized blobstore.
 // For local filesystem it uses hard links (no extra disk space).
@@ -268,18 +342,6 @@ func (is *ImageStore) upgradeToGlobalBlobstore() error {
 
 	is.log.Info().Msg("upgrading storage: populating global blobstore from existing repos")
 
-	type blobCandidate struct {
-		repoName string
-		blobPath string
-		size     int64
-	}
-
-	type repoBlobRef struct {
-		digest   godigest.Digest
-		repoName string
-		blobPath string
-	}
-
 	candidates := map[string]blobCandidate{}
 	repoBlobRefs := []repoBlobRef{}
 	promotedDigests := map[string]bool{}
@@ -324,56 +386,11 @@ func (is *ImageStore) upgradeToGlobalBlobstore() error {
 			continue
 		}
 
-		// ensure algorithm dir exists in _blobstore
-		algoDir := path.Join(is.rootDir, storageConstants.GlobalBlobsRepo,
-			ispec.ImageBlobsDir, digest.Algorithm().String())
-		if err := is.storeDriver.EnsureDir(algoDir); err != nil {
-			is.log.Error().Err(err).Str("dir", algoDir).Msg("failed to create algorithm dir")
-
+		if err := is.promoteBlobCandidate(candidate, digest, globalBlobPath); err != nil {
 			return err
 		}
 
-		if is.storeDriver.Name() == storageConstants.LocalStorageDriverName {
-			// local filesystem: use hard link (no extra disk space)
-			if err := is.storeDriver.Link(candidate.blobPath, globalBlobPath); err != nil {
-				is.log.Error().Err(err).Str("src", candidate.blobPath).Str("dst", globalBlobPath).
-					Msg("failed to link blob to global blobstore")
-
-				return err
-			}
-		} else {
-			// S3/GCS: copy the actual blob content
-			content, err := is.storeDriver.ReadFile(candidate.blobPath)
-			if err != nil {
-				is.log.Error().Err(err).Str("src", candidate.blobPath).
-					Msg("failed to read blob during upgrade")
-
-				return err
-			}
-
-			if _, err := is.storeDriver.WriteFile(globalBlobPath, content); err != nil {
-				is.log.Error().Err(err).Str("dst", globalBlobPath).
-					Msg("failed to write blob to global blobstore")
-
-				return err
-			}
-		}
-
-		// register global blobstore path as the master/original cache entry first,
-		// so that subsequent PutBlob calls for per-repo paths go into DuplicatesBucket
-		if is.cache != nil {
-			if err := is.cache.PutBlob(digest, globalBlobPath); err != nil {
-				is.log.Error().Err(err).Str("digest", digest.String()).
-					Msg("failed to update cache with global blobstore path during upgrade")
-
-				return err
-			}
-		}
-
 		promotedDigests[digest.String()] = true
-
-		is.log.Info().Str("digest", digest.String()).Str("repo", candidate.repoName).
-			Msg("upgraded blob to global blobstore")
 	}
 
 	for _, repoBlobRef := range repoBlobRefs {