diff --git a/app/logical/post_replacement_processor.rb b/app/logical/post_replacement_processor.rb
index 14b0590c6..01f4cdfa2 100644
--- a/app/logical/post_replacement_processor.rb
+++ b/app/logical/post_replacement_processor.rb
@@ -34,13 +34,14 @@ class PostReplacementProcessor
     replacement.image_width = media_asset.image_width
     replacement.md5 = media_asset.md5
 
+    post.lock!
     post.md5 = media_asset.md5
     post.file_ext = media_asset.file_ext
     post.image_width = media_asset.image_width
     post.image_height = media_asset.image_height
     post.file_size = media_asset.file_size
     post.source = replacement.final_source.presence || replacement.replacement_url
-    post.tag_string = replacement.tags
+    post.tag_string = "#{post.tag_string} #{replacement.tags}"
 
     rescale_notes(post)
     post.save!
diff --git a/app/models/post_replacement.rb b/app/models/post_replacement.rb
index 099a9c4fd..bbebbfffe 100644
--- a/app/models/post_replacement.rb
+++ b/app/models/post_replacement.rb
@@ -12,8 +12,6 @@ class PostReplacement < ApplicationRecord
 
   def initialize_fields
     self.original_url = post.source
-    self.tags = "#{post.tag_string} #{tags}"
-
     self.old_file_ext = post.file_ext
     self.old_file_size = post.file_size
     self.old_image_width = post.image_width
diff --git a/test/functional/post_replacements_controller_test.rb b/test/functional/post_replacements_controller_test.rb
index 9380dfd6e..abfddbca8 100644
--- a/test/functional/post_replacements_controller_test.rb
+++ b/test/functional/post_replacements_controller_test.rb
@@ -6,13 +6,14 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
       context "replacing a post from a source url" do
         should "replace the post" do
           assert_difference("PostReplacement.count") do
-            @post = create(:post)
+            @post = create(:post, tag_string: "image_sample")
 
             post_auth post_replacements_path, create(:moderator_user), params: {
               format: :json,
               post_id: @post.id,
               post_replacement: {
                 replacement_url: "https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg",
+                tags: "replaced -image_sample"
               }
             }
 
@@ -31,6 +32,7 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_equal(@post.file_size, @replacement.old_file_size)
           assert_equal(@post.file_ext, @replacement.old_file_ext)
           assert_equal(@post.md5, @replacement.old_md5)
+          assert_equal(@post.tag_string, "image_sample")
 
           @post.reload
           assert_equal("d34e4cf0a437a5d65f8e82b7bcd02606", @post.md5)
@@ -40,6 +42,7 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_equal(650, @post.image_height)
           assert_equal(127_238, @post.file_size)
           assert_equal("jpg", @post.file_ext)
+          assert_equal("replaced", @post.tag_string)
         end
       end