diff --git a/.changelog/unreleased/features/1287-upgrade-legacy.md b/.changelog/unreleased/features/1287-upgrade-legacy.md
new file mode 100644
index 0000000000..2ff89373cf
--- /dev/null
+++ b/.changelog/unreleased/features/1287-upgrade-legacy.md
@@ -0,0 +1,3 @@
+- Add `--legacy | -l` flag to support upgrades for chains built with Cosmos SDK < v0.43.0 ([#1287])
+
+[#1287]: https://github.com/informalsystems/ibc-rs/issues/1287
diff --git a/relayer-cli/src/commands/tx/upgrade.rs b/relayer-cli/src/commands/tx/upgrade.rs
index 8242bf5bfe..71331dfcd2 100644
--- a/relayer-cli/src/commands/tx/upgrade.rs
+++ b/relayer-cli/src/commands/tx/upgrade.rs
@@ -61,6 +61,12 @@ pub struct TxIbcUpgradeChainCmd {
         help = "a string to name the upgrade proposal plan (default: 'plan')"
     )]
     upgrade_name: Option<String>,
+
+    #[options(
+        help = "use legacy upgrade proposal constructs (for chains built with Cosmos SDK < v0.43.0)",
+        short = "l"
+    )]
+    legacy: bool,
 }
 
 impl TxIbcUpgradeChainCmd {
@@ -88,6 +94,7 @@ impl TxIbcUpgradeChainCmd {
                 .upgrade_name
                 .clone()
                 .unwrap_or_else(|| "plan".to_string()),
+            legacy: self.legacy,
         };
 
         Ok(opts)
diff --git a/relayer/src/upgrade_chain.rs b/relayer/src/upgrade_chain.rs
index 53f6df26e3..57f97b9325 100644
--- a/relayer/src/upgrade_chain.rs
+++ b/relayer/src/upgrade_chain.rs
@@ -1,7 +1,9 @@
 //! Chain upgrade plans for triggering IBC-breaking upgrades.
+#![allow(deprecated)] // TODO(hu55a1n1): remove this when we don't need legacy upgrade support
 
 use std::time::Duration;
 
+use bytes::BufMut;
 use flex_error::define_error;
 use prost_types::Any;
 
@@ -10,7 +12,7 @@ use ibc::ics02_client::height::Height;
 use ibc::ics24_host::identifier::{ChainId, ClientId};
 use ibc::{events::IbcEvent, ics07_tendermint::client_state::ClientState};
 use ibc_proto::cosmos::gov::v1beta1::MsgSubmitProposal;
-use ibc_proto::cosmos::upgrade::v1beta1::Plan;
+use ibc_proto::cosmos::upgrade::v1beta1::{Plan, SoftwareUpgradeProposal};
 use ibc_proto::ibc::core::client::v1::UpgradeProposal;
 
 use crate::chain::{ChainEndpoint, CosmosSdkChain};
@@ -55,6 +57,7 @@ pub struct UpgradePlanOptions {
     pub upgraded_chain_id: ChainId,
     pub upgraded_unbonding_period: Option<Duration>,
     pub upgrade_plan_name: String,
+    pub legacy: bool,
 }
 
 pub fn build_and_send_ibc_upgrade_proposal(
@@ -94,11 +97,16 @@ pub fn build_and_send_ibc_upgrade_proposal(
         }),
     };
 
-    let mut buf_proposal = Vec::new();
-    prost::Message::encode(&proposal, &mut buf_proposal).unwrap();
+    let proposal = if opts.legacy {
+        Proposal::Legacy(proposal.into())
+    } else {
+        Proposal::Default(proposal)
+    };
 
+    let mut buf_proposal = Vec::new();
+    proposal.encode(&mut buf_proposal);
     let any_proposal = Any {
-        type_url: "/ibc.core.client.v1.UpgradeProposal".to_string(),
+        type_url: proposal.type_url(),
         value: buf_proposal,
     };
 
@@ -138,3 +146,51 @@ pub fn build_and_send_ibc_upgrade_proposal(
         Some(reason) => Err(UpgradeChainError::tx_response(reason)),
     }
 }
+
+enum Proposal {
+    Default(UpgradeProposal),
+    Legacy(LegacyProposal),
+}
+
+impl Proposal {
+    fn encode(&self, buf: &mut impl BufMut) {
+        match self {
+            Proposal::Default(p) => prost::Message::encode(p, buf),
+            Proposal::Legacy(p) => prost::Message::encode(&p.0, buf),
+        }
+        .unwrap()
+    }
+
+    fn type_url(&self) -> String {
+        match self {
+            Proposal::Default(_) => "/ibc.core.client.v1.UpgradeProposal",
+            Proposal::Legacy(_) => "/cosmos.upgrade.v1beta1.SoftwareUpgradeProposal",
+        }
+        .to_owned()
+    }
+}
+
+struct LegacyProposal(SoftwareUpgradeProposal);
+
+impl From<UpgradeProposal> for LegacyProposal {
+    fn from(v: UpgradeProposal) -> Self {
+        let plan = {
+            if let Some(plan) = v.plan {
+                Some(Plan {
+                    name: plan.name,
+                    height: plan.height,
+                    info: plan.info,
+                    time: None,
+                    upgraded_client_state: v.upgraded_client_state,
+                })
+            } else {
+                None
+            }
+        };
+        Self(SoftwareUpgradeProposal {
+            title: v.title,
+            description: v.description,
+            plan,
+        })
+    }
+}