vaults: add backwards compatibility

Author: jamesob

bip-vaults.mediawiki

@@ -635,6 +635,18 @@ Script descriptors for vault-related outputs will be covered in a subsequent BIP
 
 TBD
 
+== Backwards compatibility ==
+
+<code>OP_VAULT</code> and <code>OP_UNVAULT</code> replace, respectively, the witness v1-only opcodes OP_SUCCESS187 and OP_SUCCESS188 with
+stricter verification semantics. Consequently, scripts using those opcodes which previously were valid will cease to be valid with this change.
+
+Stricter verification semantics for an OP_SUCCESSx opcode are a soft fork, so existing software will be fully functional without upgrade except for mining and block validation.
+
+Backwards compatibility considerations are very comparable to previous
+deployments for OP_CHECKSEQUENCEVERIFY and OP_CHECKLOCKTIMEVERIFY (see [https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki BIP-0065]
+and [https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki BIP-0112]).
+
+
 == Rationale ==
 
 <references />