diff --git a/CHANGELOG.md b/CHANGELOG.md
index 70da4a2af..f1ef0e524 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,10 @@ nav_order: 1
 <!-- Always keep the following header in place: -->
 <!-- ## [MAJOR.MINOR.PATCH] - YYYY-MM-DD -->
 
+## [MAJOR.MINOR.PATCH] - YYYY-MM-DD
+
+- Fix `aiven_project`: can't migrate from `account_id` (deprecated) to `parent_id`
+
 ## [4.27.0] - 2024-10-09
 
 - Remove `aiven_thanos` from beta resources
diff --git a/internal/sdkprovider/service/project/project.go b/internal/sdkprovider/service/project/project.go
index d2372408c..cce4798e8 100644
--- a/internal/sdkprovider/service/project/project.go
+++ b/internal/sdkprovider/service/project/project.go
@@ -30,12 +30,9 @@ var aivenProjectSchema = map[string]*schema.Schema{
 	"account_id": {
 		Type:        schema.TypeString,
 		Optional:    true,
+		Computed:    true,
 		Description: userconfig.Desc("Link a project to an existing account using its account ID. This field is deprecated. Use `parent_id` instead.").Referenced().Build(),
 		Deprecated:  "Use parent_id instead. This field will be removed in the next major release.",
-		DiffSuppressFunc: func(_, _, _ string, d *schema.ResourceData) bool {
-			_, ok := d.GetOk("parent_id")
-			return ok
-		},
 	},
 	"parent_id": {
 		Type:     schema.TypeString,
@@ -43,10 +40,6 @@ var aivenProjectSchema = map[string]*schema.Schema{
 		Description: userconfig.Desc(
 			"Link a project to an [organization, organizational unit,](https://aiven.io/docs/platform/concepts/orgs-units-projects) or account by using its ID.",
 		).Referenced().Build(),
-		DiffSuppressFunc: func(_, _, _ string, d *schema.ResourceData) bool {
-			_, ok := d.GetOk("account_id")
-			return ok
-		},
 	},
 	"copy_from_project": {
 		Type:             schema.TypeString,
@@ -438,7 +431,7 @@ func setProjectTerraformProperties(
 	client *aiven.Client,
 	project *aiven.Project,
 ) diag.Diagnostics {
-	if stateID, ok := d.GetOk("parent_id"); ok {
+	if stateID := d.Get("parent_id"); stateID != "" {
 		idToSet, err := schemautil.DetermineMixedOrganizationConstraintIDToStore(
 			ctx,
 			client,
diff --git a/internal/sdkprovider/service/project/project_test.go b/internal/sdkprovider/service/project/project_test.go
index 158595fd9..407ed7903 100644
--- a/internal/sdkprovider/service/project/project_test.go
+++ b/internal/sdkprovider/service/project/project_test.go
@@ -59,18 +59,27 @@ func TestAccAivenProject_accounts(t *testing.T) {
 	resourceName := "aiven_project.foo"
 	rName := acctest.RandStringFromCharSet(10, acctest.CharSetAlphaNum)
 
+	config := testAccProjectResourceAccounts(rName)
 	resource.ParallelTest(t, resource.TestCase{
 		PreCheck:                 func() { acc.TestAccPreCheck(t) },
 		ProtoV6ProviderFactories: acc.TestProtoV6ProviderFactories,
 		CheckDestroy:             testAccCheckAivenProjectResourceDestroy,
 		Steps: []resource.TestStep{
 			{
-				Config: testAccProjectResourceAccounts(rName),
+				Config: config,
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckAivenProjectAttributes("data.aiven_project.project", "account_id"),
 					resource.TestCheckResourceAttr(resourceName, "project", fmt.Sprintf("test-acc-pr-%s", rName)),
 				),
 			},
+			{
+				// Tests account_id (deprecated) -> parent_id migration
+				Config: regexp.MustCompile(`account_id\s+=`).ReplaceAllString(config, "parent_id ="),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttrSet(resourceName, "account_id"), // it is computed and should be set
+					resource.TestCheckResourceAttrSet(resourceName, "parent_id"),
+				),
+			},
 		},
 	})
 }