diff --git a/src/backend/distributed/sql/citus--11.3-1--12.0-1.sql b/src/backend/distributed/sql/citus--11.3-1--12.0-1.sql
index 998ffc2be..a35d772b7 100644
--- a/src/backend/distributed/sql/citus--11.3-1--12.0-1.sql
+++ b/src/backend/distributed/sql/citus--11.3-1--12.0-1.sql
@@ -42,3 +42,10 @@ DROP FUNCTION citus_shard_sizes;
 
 #include "udfs/drop_old_time_partitions/12.0-1.sql"
 #include "udfs/get_missing_time_partition_ranges/12.0-1.sql"
+
+-- Update the default rebalance strategy to 'by_disk_size', but only if the
+-- default is currently 'by_shard_count'
+SELECT citus_set_default_rebalance_strategy(name)
+FROM pg_dist_rebalance_strategy
+WHERE name = 'by_disk_size'
+    AND (SELECT default_strategy FROM pg_dist_rebalance_strategy WHERE name = 'by_shard_count');
diff --git a/src/backend/distributed/sql/downgrades/citus--12.0-1--11.3-1.sql b/src/backend/distributed/sql/downgrades/citus--12.0-1--11.3-1.sql
index 1adb4cb72..4a25cfda0 100644
--- a/src/backend/distributed/sql/downgrades/citus--12.0-1--11.3-1.sql
+++ b/src/backend/distributed/sql/downgrades/citus--12.0-1--11.3-1.sql
@@ -76,3 +76,8 @@ DROP FUNCTION pg_catalog.citus_stat_tenants_local_internal(
 
 #include "../udfs/drop_old_time_partitions/10.2-1.sql"
 #include "../udfs/get_missing_time_partition_ranges/10.2-1.sql"
+
+-- This explicitly does not reset the rebalance strategy to by_shard_count,
+-- because there's no way of knowing if the rebalance strategy before the
+-- upgrade was by_disk_size or by_shard_count. And even in previous versions
+-- by_disk_size is considered superior for quite some time.
diff --git a/src/test/regress/expected/shard_rebalancer.out b/src/test/regress/expected/shard_rebalancer.out
index 62ae17487..b8f4010b1 100644
--- a/src/test/regress/expected/shard_rebalancer.out
+++ b/src/test/regress/expected/shard_rebalancer.out
@@ -3,6 +3,14 @@
 --
 SET citus.next_shard_id TO 433000;
 SET citus.propagate_session_settings_for_loopback_connection TO ON;
+-- Because of historic reasons this test was written in a way that assumes that
+-- by_shard_count is the default strategy.
+SELECT citus_set_default_rebalance_strategy('by_shard_count');
+ citus_set_default_rebalance_strategy
+---------------------------------------------------------------------
+
+(1 row)
+
 -- Lower the minimum disk size that a shard group is considered as. Otherwise
 -- we need to create shards of more than 100MB.
 ALTER SYSTEM SET citus.rebalancer_by_disk_size_base_cost = 0;
@@ -2863,6 +2871,12 @@ select 1 from citus_add_node('localhost', :worker_2_port);
 select rebalance_table_shards();
 ERROR:  cannot use logical replication to transfer shards of the relation table_without_primary_key since it doesn't have a REPLICA IDENTITY or PRIMARY KEY
 DROP TABLE table_with_primary_key, table_without_primary_key;
+SELECT citus_set_default_rebalance_strategy('by_disk_size');
+ citus_set_default_rebalance_strategy
+---------------------------------------------------------------------
+
+(1 row)
+
 ALTER SYSTEM RESET citus.rebalancer_by_disk_size_base_cost;
 SELECT pg_reload_conf();
  pg_reload_conf
diff --git a/src/test/regress/expected/single_shard_table_udfs.out b/src/test/regress/expected/single_shard_table_udfs.out
index 3b73473b0..d49027b60 100644
--- a/src/test/regress/expected/single_shard_table_udfs.out
+++ b/src/test/regress/expected/single_shard_table_udfs.out
@@ -334,7 +334,7 @@ ERROR:  Table 'single_shard_table_col2_1' is streaming replicated. Shards of str
 SELECT citus_copy_shard_placement(1820005, :worker_1_node, :worker_2_node);
 ERROR:  Table 'single_shard_table_col2_1' is streaming replicated. Shards of streaming replicated tables cannot be copied
 -- no changes because it's already balanced
-SELECT rebalance_table_shards();
+SELECT rebalance_table_shards(rebalance_strategy := 'by_shard_count');
  rebalance_table_shards
 ---------------------------------------------------------------------
 
diff --git a/src/test/regress/sql/shard_rebalancer.sql b/src/test/regress/sql/shard_rebalancer.sql
index ba22a8abd..d64fb6826 100644
--- a/src/test/regress/sql/shard_rebalancer.sql
+++ b/src/test/regress/sql/shard_rebalancer.sql
@@ -5,6 +5,9 @@
 SET citus.next_shard_id TO 433000;
 SET citus.propagate_session_settings_for_loopback_connection TO ON;
 
+-- Because of historic reasons this test was written in a way that assumes that
+-- by_shard_count is the default strategy.
+SELECT citus_set_default_rebalance_strategy('by_shard_count');
 -- Lower the minimum disk size that a shard group is considered as. Otherwise
 -- we need to create shards of more than 100MB.
 ALTER SYSTEM SET citus.rebalancer_by_disk_size_base_cost = 0;
@@ -1574,6 +1577,7 @@ select 1 from citus_add_node('localhost', :worker_2_port);
 select rebalance_table_shards();
 
 DROP TABLE table_with_primary_key, table_without_primary_key;
+SELECT citus_set_default_rebalance_strategy('by_disk_size');
 ALTER SYSTEM RESET citus.rebalancer_by_disk_size_base_cost;
 SELECT pg_reload_conf();
 \c - - - :worker_1_port
diff --git a/src/test/regress/sql/single_shard_table_udfs.sql b/src/test/regress/sql/single_shard_table_udfs.sql
index 615e566db..7566f53e3 100644
--- a/src/test/regress/sql/single_shard_table_udfs.sql
+++ b/src/test/regress/sql/single_shard_table_udfs.sql
@@ -160,7 +160,7 @@ SELECT master_copy_shard_placement(1820005, 'localhost', :worker_1_port, 'localh
 SELECT citus_copy_shard_placement(1820005, :worker_1_node, :worker_2_node);
 
 -- no changes because it's already balanced
-SELECT rebalance_table_shards();
+SELECT rebalance_table_shards(rebalance_strategy := 'by_shard_count');
 
 -- same placements
 SELECT shardid, nodeport FROM pg_dist_shard_placement WHERE shardid > 1820000 ORDER BY shardid;