diff --git a/stackit/internal/services/mongodbflex/instance/resource.go b/stackit/internal/services/mongodbflex/instance/resource.go
index 2a3b8ca0..47d8ddae 100644
--- a/stackit/internal/services/mongodbflex/instance/resource.go
+++ b/stackit/internal/services/mongodbflex/instance/resource.go
@@ -250,6 +250,9 @@ func (r *instanceResource) Schema(_ context.Context, _ resource.SchemaRequest, r
 			},
 			"replicas": schema.Int64Attribute{
 				Required: true,
+				PlanModifiers: []planmodifier.Int64{
+					int64planmodifier.RequiresReplace(),
+				},
 			},
 			"storage": schema.SingleNestedAttribute{
 				Required: true,