diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2278,15 +2278,28 @@
     // If this block is deactivating a fork, we move all mempool transactions
     // in front of disconnectpool for reprocessing in a future
     // updateMempoolForReorg call
-    if (pindexDelete->pprev != nullptr &&
-        GetNextBlockScriptFlags(consensusParams, pindexDelete) !=
-            GetNextBlockScriptFlags(consensusParams, pindexDelete->pprev)) {
-        LogPrint(BCLog::MEMPOOL,
-                 "Disconnecting mempool due to rewind of upgrade block\n");
-        if (disconnectpool) {
-            disconnectpool->importMempool(g_mempool);
+    if (pindexDelete->pprev != nullptr) {
+        bool changes = false;
+        changes |=
+            (GetNextBlockScriptFlags(consensusParams, pindexDelete) !=
+             GetNextBlockScriptFlags(consensusParams, pindexDelete->pprev));
+
+        changes |=
+            (GetDefaultAncestorLimit(consensusParams, pindexDelete) !=
+             GetDefaultAncestorLimit(consensusParams, pindexDelete->pprev));
+
+        changes |=
+            (GetDefaultDescendantLimit(consensusParams, pindexDelete) !=
+             GetDefaultDescendantLimit(consensusParams, pindexDelete->pprev));
+
+        if (changes) {
+            LogPrint(BCLog::MEMPOOL,
+                     "Disconnecting mempool due to rewind of upgrade block\n");
+            if (disconnectpool) {
+                disconnectpool->importMempool(g_mempool);
+            }
+            g_mempool.clear();
         }
-        g_mempool.clear();
     }
 
     if (disconnectpool) {
diff --git a/test/functional/abc-phonon-mempoolpolicy.py b/test/functional/abc-phonon-mempoolpolicy.py
new file mode 100755
--- /dev/null
+++ b/test/functional/abc-phonon-mempoolpolicy.py
@@ -0,0 +1,100 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 The Bitcoin developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+"""Test that long chains are trimmed from mempool on phonon-rollback.
+
+This test does not cover that the limits are enforced by sendrawtransaction,
+the test mempool_packages.py covers that.
+
+This test can be deleted after new policy takes effect."""
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    satoshi_round,
+)
+
+from decimal import Decimal
+
+ACTIVATION_TIME = 2000000000
+"""Phonos dummy activation time"""
+
+REPLAY_PROTECTION_TIME = ACTIVATION_TIME * 2
+"""Replay protection time needs to be moved beyond phonos activation"""
+
+PREFORK_MAX_ANCESTORS = 25
+POSTFORK_MAX_ANCESTORS = 50
+
+
+def mine_until_mtp(node, new_mtp):
+    """Generates blocks until new mtp is reached"""
+    node.setmocktime(new_mtp)
+
+    while True:
+        current_mtp = node.getblockheader(
+            node.getbestblockhash())['mediantime']
+        assert(current_mtp <= new_mtp)
+        if current_mtp == new_mtp:
+            break
+        node.generate(1)
+
+
+class PhononMempoolPolicyTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+
+        self.extra_args = [[
+            "-phononactivationtime={}".format(ACTIVATION_TIME),
+            "-replayprotectionactivationtime={}".format(
+                REPLAY_PROTECTION_TIME)]]
+        # "-maxorphantx=1000"]]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def chain_transaction(self, node, parent_txid, vout, value):
+        """Build a transaction that spends parent_txid:vout. Return amount sent"""
+        fee = Decimal(0.0001)
+        send_value = satoshi_round(value - fee)
+        inputs = [{'txid': parent_txid, 'vout': vout}]
+        outputs = {node.getnewaddress(): send_value}
+        rawtx = node.createrawtransaction(inputs, outputs)
+        signedtx = node.signrawtransactionwithwallet(rawtx)
+        txid = node.sendrawtransaction(signedtx['hex'])
+        fulltx = node.getrawtransaction(txid, 1)
+        # make sure we didn't generate a change output
+        assert len(fulltx['vout']) == 1
+        return (txid, send_value)
+
+    def run_test(self):
+        # Mine some pre-fork blocks
+        node = self.nodes[0]
+        mine_until_mtp(node, ACTIVATION_TIME - 1)
+        pre_fork_height = node.getblockcount()
+
+        # Activate phonon
+        mine_until_mtp(node, ACTIVATION_TIME)
+
+        # Mine a POSTFORK_MAX_ANCESTORS length chain
+        assert_equal(0, len(node.getrawmempool(True)))
+        utxo = self.nodes[0].listunspent(1)[0]
+        txid = utxo['txid']
+        value = utxo['amount']
+
+        for _ in range(POSTFORK_MAX_ANCESTORS):
+            (txid, value) = self.chain_transaction(
+                node, txid, vout=0, value=value)
+
+        assert_equal(POSTFORK_MAX_ANCESTORS, len(node.getrawmempool(True)))
+
+        # Rollback the fork
+        node.invalidateblock(node.getblockhash(pre_fork_height + 1))
+
+        # Assert that the mempool was trimmed to PREFORK_MAX_ANCESTORS
+        assert_equal(PREFORK_MAX_ANCESTORS, len(node.getrawmempool(True)))
+
+
+if __name__ == '__main__':
+    PhononMempoolPolicyTest().main()